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/helpers.py

@@ -1,6 +1,6 @@
 import os
 from datetime import datetime, timezone
-from typing import Literal, Sequence
+from typing import Sequence
 
 
 def select_best_live_path(states: Sequence) -> str:
@@ -23,13 +23,6 @@ def select_best_live_path(states: Sequence) -> str:
     return alive[0].file_path
 
 
-ALLOWED_ROOTS: tuple[Literal["models", "input", "output"], ...] = (
-    "models",
-    "input",
-    "output",
-)
-
-
 def escape_sql_like_string(s: str, escape: str = "!") -> tuple[str, str]:
     """Escapes %, _ and the escape char in a LIKE prefix.