feat: add legacy key/data file migration to database

- Add migration_legacy_files_completed flag to SystemSettings model
- Create legacy_file_migration service to migrate series from key/data files
- Integrate legacy migration into initialization_service startup flow
- Add integration tests for legacy file migration
- Update DATABASE.md documentation with migration details
- Fix various test and service issues (nfo_repair, tmdb_client, download_service)
- Add test_database_schema unit tests

src/server/database/models.py

@@ -83,6 +83,10 @@ class AnimeSeries(Base, TimestampMixin):
         Boolean, nullable=False, default=False, server_default="0",
         doc="Whether tvshow.nfo file exists for this series"
     )
+    nfo_path: Mapped[Optional[str]] = mapped_column(
+        String(1000), nullable=True,
+        doc="Path to the tvshow.nfo metadata file"
+    )
     nfo_created_at: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True), nullable=True,
         doc="Timestamp when NFO was first created"
@@ -91,6 +95,7 @@ class AnimeSeries(Base, TimestampMixin):
         DateTime(timezone=True), nullable=True,
         doc="Timestamp when NFO was last updated"
     )
+    # TMDB (The Movie Database) ID for series metadata
     tmdb_id: Mapped[Optional[int]] = mapped_column(
         Integer, nullable=True, index=True,
         doc="TMDB (The Movie Database) ID for series metadata"
@@ -608,6 +613,10 @@ class SystemSettings(Base, TimestampMixin):
         Boolean, nullable=False, default=False, server_default="0",
         doc="Whether the initial media scan has been completed"
     )
+    migration_legacy_files_completed: Mapped[bool] = mapped_column(
+        Boolean, nullable=False, default=False, server_default="0",
+        doc="Whether legacy key/data file migration has been completed"
+    )
     last_scan_timestamp: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True), nullable=True,
         doc="Timestamp of the last completed scan"