docs: Update API, CHANGELOG, and ARCHITECTURE for Enhanced Anime Add Flow

docs/API.md

@@ -268,7 +268,7 @@ Source: [src/server/api/anime.py](../src/server/api/anime.py#L477-L495)
 
 ### POST /api/anime/add
 
-Add a new series to the library.
+Add a new series to the library with automatic database persistence, folder creation, and episode scanning.
 
 **Authentication:** Required
 
@@ -289,10 +289,28 @@ Add a new series to the library.
     "message": "Successfully added series: Attack on Titan",
     "key": "attack-on-titan",
     "folder": "Attack on Titan",
-    "db_id": 1
+    "db_id": 1,
+    "missing_episodes": ["1-1", "1-2", "1-3"],
+    "total_missing": 3
 }
 ```
 
+**Enhanced Flow:**
+
+1. Validates the request (link format, name)
+2. Creates Serie object with sanitized folder name
+3. Saves to database via AnimeDBService
+4. Creates folder using sanitized display name (not internal key)
+5. Performs targeted episode scan for this anime only
+6. Returns response with missing episodes count
+
+**Folder Name Sanitization:**
+
+- Removes invalid filesystem characters: `< > : " / \ | ? *`
+- Trims leading/trailing whitespace and dots
+- Preserves Unicode characters (for Japanese titles)
+- Example: `"Attack on Titan: Final Season"` → `"Attack on Titan Final Season"`
+
 Source: [src/server/api/anime.py](../src/server/api/anime.py#L604-L710)
 
 ### POST /api/anime/rescan