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>

alembic_db/versions/0002_add_is_missing_to_cache_state.py

@@ -0,0 +1,37 @@
+"""
+Add is_missing column to asset_cache_state for non-destructive soft-delete
+
+Revision ID: 0002_add_is_missing
+Revises: 0001_assets
+Create Date: 2025-02-05 00:00:00
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0002_add_is_missing"
+down_revision = "0001_assets"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "asset_cache_state",
+        sa.Column(
+            "is_missing",
+            sa.Boolean(),
+            nullable=False,
+            server_default=sa.text("false"),
+        ),
+    )
+    op.create_index(
+        "ix_asset_cache_state_is_missing",
+        "asset_cache_state",
+        ["is_missing"],
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("ix_asset_cache_state_is_missing", table_name="asset_cache_state")
+    op.drop_column("asset_cache_state", "is_missing")