fix: address code review feedback - round 2

- Reject path separators (/, \, os.sep) in tag components for defense-in-depth
- Add comment explaining double-relpath normalization trick
- Add _require_assets_feature_enabled decorator returning 503 when disabled
- Call asset_seeder.disable() when --enable-assets is not passed
- Add iter_chunks to bulk_update_needs_verify, bulk_update_is_missing,
  and delete_references_by_ids to respect SQLite bind param limits
- Fix CacheStateRow.size_bytes NULL coercion (0 -> None) to avoid
  false needs_verify flags on assets with unknown size
- Add PermissionError catch in delete_asset_tags route (403 vs 500)
- Add hash-is-None guard in delete_orphaned_seed_asset
- Validate from_asset_id in reassign_asset_references
- Initialize _prune_first in __init__, remove getattr workaround
- Cap error accumulation in _add_error to 200
- Remove confirmed dead code: seed_assets, compute_filename_for_asset,
  ALLOWED_ROOTS, AssetNotFoundError, SetTagsResult, update_enrichment_level,
  Asset.to_dict, AssetReference.to_dict, _AssetSeeder.enable

Amp-Thread-ID: https://ampcode.com/threads/T-019cb610-1b55-74b6-8dbb-381d73c387c0
Co-authored-by: Amp <amp@ampcode.com>

app/assets/database/models.py

@@ -20,7 +20,7 @@ from sqlalchemy import (
 from sqlalchemy.orm import Mapped, foreign, mapped_column, relationship
 
 from app.assets.helpers import get_utc_now
-from app.database.models import Base, to_dict
+from app.database.models import Base
 
 
 class Asset(Base):
@@ -59,9 +59,6 @@ class Asset(Base):
         CheckConstraint("size_bytes >= 0", name="ck_assets_size_nonneg"),
     )
 
-    def to_dict(self, include_none: bool = False) -> dict[str, Any]:
-        return to_dict(self, include_none=include_none)
-
     def __repr__(self) -> str:
         return f"<Asset id={self.id} hash={(self.hash or '')[:12]}>"
 
@@ -161,11 +158,6 @@ class AssetReference(Base):
         ),
     )
 
-    def to_dict(self, include_none: bool = False) -> dict[str, Any]:
-        data = to_dict(self, include_none=include_none)
-        data["tags"] = [t.name for t in self.tags]
-        return data
-
     def __repr__(self) -> str:
         path_part = f" path={self.file_path!r}" if self.file_path else ""
         return f"<AssetReference id={self.id} name={self.name!r}{path_part}>"