feat: non-destructive asset pruning with is_missing flag

- Add is_missing column to AssetCacheState for soft-delete
- Replace hard-delete pruning with mark_cache_states_missing_outside_prefixes
- Auto-restore missing cache states when files are re-scanned
- Filter out missing cache states from queries by default
- Rename functions for clarity:
  - mark_cache_states_missing_outside_prefixes (was delete_cache_states_outside_prefixes)
  - get_unreferenced_unhashed_asset_ids (was get_orphaned_seed_asset_ids)
  - mark_assets_missing_outside_prefixes (was prune_orphaned_assets)
  - mark_missing_outside_prefixes_safely (was prune_orphans_safely)
- Add restore_cache_states_by_paths for explicit restoration
- Add cleanup_unreferenced_assets for explicit hard-delete when needed
- Update API endpoint /api/assets/prune to use new soft-delete behavior

This preserves user metadata (tags, etc.) when base directories change,
allowing assets to be restored when the original paths become available again.

Amp-Thread-ID: https://ampcode.com/threads/T-019c3114-bf28-73a9-a4d2-85b208fd5462
Co-authored-by: Amp <amp@ampcode.com>

app/assets/database/models.py

@@ -83,12 +83,14 @@ class AssetCacheState(Base):
     file_path: Mapped[str] = mapped_column(Text, nullable=False)
     mtime_ns: Mapped[int | None] = mapped_column(BigInteger, nullable=True)
     needs_verify: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
+    is_missing: Mapped[bool] = mapped_column(Boolean, nullable=False, default=False)
 
     asset: Mapped[Asset] = relationship(back_populates="cache_states")
 
     __table_args__ = (
         Index("ix_asset_cache_state_file_path", "file_path"),
         Index("ix_asset_cache_state_asset_id", "asset_id"),
+        Index("ix_asset_cache_state_is_missing", "is_missing"),
         CheckConstraint(
             "(mtime_ns IS NULL) OR (mtime_ns >= 0)", name="ck_acs_mtime_nonneg"
         ),