feat(setup): track unresolved folders for manual key resolution

When SetupService cannot auto-resolve a provider key for an anime folder,
the folder is now tracked in the new 'unresolved_folders' table instead of
being silently skipped. Users can then resolve these via the new API:

- GET /api/setup/unresolved - list unresolved folders with search suggestions
- POST /api/setup/unresolved/{folder}/resolve - provide key to resolve folder

The SetupService.run() now:
- Tracks unresolved folders instead of skipping them
- Re-creates AnimeSeries for previously unresolved folders that are now resolved
- Includes unresolved count in logs

New files:
- src/server/api/setup_endpoints.py - API endpoints for unresolved management
- tests/unit/test_unresolved_folder_service.py - service and model tests

Modified:
- src/server/database/models.py - add UnresolvedFolder model
- src/server/database/service.py - add UnresolvedFolderService
- src/server/services/setup_service.py - track unresolved folders
- src/server/fastapi_app.py - include setup router

src/server/database/models.py

@@ -626,6 +626,96 @@ class UserSession(Base, TimestampMixin):
         self.is_active = False
 
 
+class UnresolvedFolder(Base, TimestampMixin):
+    """SQLAlchemy model for folders that couldn't be resolved during setup.
+    
+    Tracks anime folders whose provider key couldn't be auto-resolved
+    during the initial setup scan. Users can provide the correct key
+    via the API to complete the series registration.
+    
+    Attributes:
+        id: Primary key
+        folder_name: Original filesystem folder name
+        title: Extracted title from folder name
+        year: Extracted release year (optional)
+        provider_key: User-provided provider key to resolve this folder
+        search_attempts: Number of auto-search attempts made
+        last_search_result: Cached search results (JSON string) for UI suggestions
+        resolved_at: Timestamp when provider_key was provided
+        created_at: Creation timestamp (from TimestampMixin)
+        updated_at: Last update timestamp (from TimestampMixin)
+    """
+    __tablename__ = "unresolved_folders"
+    
+    # Primary key
+    id: Mapped[int] = mapped_column(
+        Integer, primary_key=True, autoincrement=True
+    )
+    
+    # Folder metadata
+    folder_name: Mapped[str] = mapped_column(
+        String(1000), unique=True, nullable=False, index=True,
+        doc="Original filesystem folder name"
+    )
+    title: Mapped[str] = mapped_column(
+        String(500), nullable=False,
+        doc="Extracted title from folder name"
+    )
+    year: Mapped[Optional[int]] = mapped_column(
+        Integer, nullable=True,
+        doc="Extracted release year"
+    )
+    
+    # Resolution data
+    provider_key: Mapped[Optional[str]] = mapped_column(
+        String(255), nullable=True,
+        doc="User-provided provider key to resolve this folder"
+    )
+    search_attempts: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0, server_default="0",
+        doc="Number of auto-search attempts made"
+    )
+    last_search_result: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True,
+        doc="Cached search results (JSON) for UI display"
+    )
+    resolved_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True,
+        doc="Timestamp when this folder was resolved"
+    )
+    
+    @validates('folder_name')
+    def validate_folder_name(self, key: str, value: str) -> str:
+        """Validate folder name is not empty."""
+        if not value or not value.strip():
+            raise ValueError("Folder name cannot be empty")
+        if len(value) > 1000:
+            raise ValueError("Folder name must be 1000 characters or less")
+        return value.strip()
+    
+    @validates('title')
+    def validate_title(self, key: str, value: str) -> str:
+        """Validate title is not empty."""
+        if not value or not value.strip():
+            raise ValueError("Title cannot be empty")
+        if len(value) > 500:
+            raise ValueError("Title must be 500 characters or less")
+        return value.strip()
+    
+    @property
+    def is_resolved(self) -> bool:
+        """Check if this folder has been resolved with a provider key."""
+        return self.provider_key is not None and self.resolved_at is not None
+    
+    def __repr__(self) -> str:
+        return (
+            f"<UnresolvedFolder(id={self.id}, "
+            f"folder_name='{self.folder_name}', "
+            f"title='{self.title}', "
+            f"resolved={self.is_resolved})>"
+        )
+
+
 class SystemSettings(Base, TimestampMixin):
     """SQLAlchemy model for system-wide settings and state.