fix: merge bare folder into year-suffixed one instead of bailing

When the year-suffixed target folder already exists on disk, both
FolderNamingService and AnimeService.rename_folder_if_needed used to
silently bail out. The bare folder (e.g. 'Ultraman') was left next to
the year-suffixed one ('Ultraman (2019)'), producing the symptom
'reports series like Ultraman as added twice' — the DB has one row but
the filesystem has two folders holding the same content.

Fix: when the target already exists, merge the source's contents into
the target (target version wins on file conflicts; source copies are
removed so cleanup succeeds), remove the now-empty source directory,
update DB row + in-memory cache. Plain rename path is unchanged.

Also fixes a latent TypeError in rename_folder_if_needed where
self._directory (a str) was used with the '/' operator. Production
behavior was that any rename through that method raised and was
swallowed by the caller's try/except, leaving the bare folder
untouched. The new path builds Path objects from the string base.

Tests:
- Replaced test_skips_when_target_folder_already_exists (which
  codified the bug) with three tests that cover the new merge
  contract: clean merge, no-overwrite, and empty-source removal.
- Added tests/unit/test_rename_folder_if_needed.py covering the
  same scenarios plus the str-directory regression. All seven go
  red on the unfixed code and green with the fix.

Fixes the 'Ultraman' / 'Ultraman (2019)' duplicate-folder bug.

src/server/services/anime_service.py

@@ -1244,8 +1244,7 @@ class AnimeService:
         Returns:
             True if rename was performed, False if no rename needed or failed
         """
-        import os
-        import shutil
+        from pathlib import Path
 
         if current_folder == target_folder:
             logger.debug(
@@ -1254,8 +1253,9 @@ class AnimeService:
             )
             return False
 
-        current_path = self._directory / current_folder
-        target_path = self._directory / target_folder
+        base_dir = Path(self._directory)
+        current_path = base_dir / current_folder
+        target_path = base_dir / target_folder
 
         if not current_path.exists():
             logger.debug(
@@ -1265,15 +1265,54 @@ class AnimeService:
             return False
 
         if target_path.exists():
-            logger.warning(
-                "Cannot rename folder for %s: target path already exists: %s",
-                key,
-                target_path
+            # Target already exists — merge source into target instead of
+            # bailing. Without this, a bare folder ('Naruto') next to the
+            # year-suffixed one ('Naruto (2019)') would orphan the bare
+            # folder forever, producing the "series added twice" symptom.
+            try:
+                summary = self._merge_folder_into_target(
+                    str(current_path), str(target_path)
+                )
+            except Exception as exc:
+                logger.error(
+                    "Failed to merge %s -> %s for %s: %s",
+                    current_folder, target_folder, key, exc,
+                )
+                return False
+            logger.info(
+                "Merged folder %s -> %s for series %s (moved=%d skipped=%d removed_source=%s)",
+                current_folder, target_folder, key,
+                summary["moved"], summary["skipped"], summary["removed_source"],
             )
-            return False
+
+            # Update in-memory cache
+            if key in self._app.list.keyDict:
+                self._app.list.keyDict[key].folder = target_folder
+                logger.debug(
+                    "Updated in-memory cache folder for %s: %s",
+                    key, target_folder
+                )
+
+            # Update database if session provided
+            if db is not None:
+                from src.server.database.service import AnimeSeriesService
+
+                # Look up series by key to get database ID
+                series = await AnimeSeriesService.get_by_key(db, key)
+                if series:
+                    await AnimeSeriesService.update(
+                        db, series_id=series.id, folder=target_folder
+                    )
+                    logger.debug(
+                        "Updated DB folder for %s: %s",
+                        key, target_folder
+                    )
+
+            return True
 
         try:
             # Rename folder on disk
+            import shutil
             shutil.move(str(current_path), str(target_path))
             logger.info(
                 "Renamed folder for %s: %s -> %s",
@@ -1317,6 +1356,83 @@ class AnimeService:
             )
             return False
 
+    @staticmethod
+    def _merge_folder_into_target(source: str, target: str) -> dict:
+        """Merge a source folder's contents into an existing target folder.
+
+        Walks the source tree and moves every file into the matching path
+        under the target. When a destination file already exists, the
+        source copy is removed (the target version wins; we don't keep
+        duplicates). When the source tree is fully consumed, the
+        (now-empty) source directory is removed.
+
+        Both paths must be absolute and ``target`` must already exist on
+        disk.
+
+        Returns a summary dict with ``moved`` (file count), ``skipped``
+        (file count where target already had a copy), and
+        ``removed_source`` (bool).
+        """
+        import os
+        import shutil
+
+        if not os.path.isdir(source):
+            return {"moved": 0, "skipped": 0, "removed_source": False}
+        if not os.path.isdir(target):
+            raise ValueError(f"target does not exist: {target}")
+
+        moved = 0
+        skipped = 0
+        for root, _dirs, files in os.walk(source):
+            rel_root = os.path.relpath(root, source)
+            dest_root = (
+                target if rel_root == "."
+                else os.path.join(target, rel_root)
+            )
+            os.makedirs(dest_root, exist_ok=True)
+            for name in files:
+                src_file = os.path.join(root, name)
+                dest_file = os.path.join(dest_root, name)
+                if os.path.exists(dest_file):
+                    # Target wins — never overwrite existing content.
+                    # Remove the orphaned source copy so cleanup below
+                    # can rmdir it.
+                    try:
+                        os.remove(src_file)
+                    except OSError as exc:
+                        logger.warning(
+                            "merge: could not remove duplicate %s: %s",
+                            src_file, exc,
+                        )
+                    skipped += 1
+                    logger.warning(
+                        "merge: skipping %s (target already has %s)",
+                        src_file, dest_file,
+                    )
+                    continue
+                shutil.move(src_file, dest_file)
+                moved += 1
+
+        # Try to remove the (now empty) source tree. Walk bottom-up so
+        # leaf directories are removed before their parents.
+        removed_source = False
+        for root, dirs, files in os.walk(source, topdown=False):
+            for d in dirs:
+                try:
+                    os.rmdir(os.path.join(root, d))
+                except OSError:
+                    pass
+        try:
+            os.rmdir(source)
+            removed_source = True
+        except OSError as exc:
+            logger.warning(
+                "merge: could not remove source directory %s: %s",
+                source, exc,
+            )
+
+        return {"moved": moved, "skipped": skipped, "removed_source": removed_source}
+
     async def contains_in_db(self, key: str, db) -> bool:
         """
         Check if a series with the given key exists in the database.