refactor: rename functions to verb-based naming convention

Rename functions across app/assets/ to follow verb-based naming:
- is_scalar → check_is_scalar
- project_kv → expand_metadata_to_rows
- _visible_owner_clause → _build_visible_owner_clause
- _chunk_rows → _iter_row_chunks
- _at_least_one → _validate_at_least_one_field
- _tags_norm → _normalize_tags_field
- _ser_dt → _serialize_datetime
- _ser_updated → _serialize_updated_at
- _error_response → _build_error_response
- _validation_error_response → _build_validation_error_response
- file_sender → stream_file_chunks
- seed_assets_endpoint → seed_assets
- utcnow → get_utc_now
- _safe_sort_field → _validate_sort_field
- _safe_filename → _sanitize_filename
- fast_asset_file_check → check_asset_file_fast
- prefixes_for_root → get_prefixes_for_root
- blake3_hash → compute_blake3_hash
- blake3_hash_async → compute_blake3_hash_async
- _is_within → _check_is_within
- _rel → _compute_relative

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/assets/api/schemas_in.py

@@ -108,7 +108,7 @@ class UpdateAssetBody(BaseModel):
     user_metadata: dict[str, Any] | None = None
 
     @model_validator(mode="after")
-    def _at_least_one(self):
+    def _validate_at_least_one_field(self):
         if self.name is None and self.user_metadata is None:
             raise ValueError("Provide at least one of: name, user_metadata.")
         return self
@@ -137,7 +137,7 @@ class CreateFromHashBody(BaseModel):
 
     @field_validator("tags", mode="before")
     @classmethod
-    def _tags_norm(cls, v):
+    def _normalize_tags_field(cls, v):
         if v is None:
             return []
         if isinstance(v, list):