feat: implement two-phase scanning architecture (fast + enrich)

Phase 1 (FAST): Creates stub records with filesystem metadata only
- path, size, mtime - no file content reading
- Populates asset database quickly on startup

Phase 2 (ENRICH): Extracts metadata and computes hashes
- Safetensors header parsing, MIME types
- Optional blake3 hash computation
- Updates existing stub records

Changes:
- Add ScanPhase enum (FAST, ENRICH, FULL)
- Add enrichment_level column to AssetCacheState (0=stub, 1=metadata, 2=hashed)
- Add build_stub_specs() for fast scanning without metadata extraction
- Add get_unenriched_cache_states(), enrich_asset(), enrich_assets_batch()
- Add start_fast(), start_enrich() convenience methods to AssetSeeder
- Update start() to accept phase parameter (defaults to FULL)
- Split _run_scan() into _run_fast_phase() and _run_enrich_phase()
- Add migration 0003_add_enrichment_level.py
- Update tests for new architecture

Amp-Thread-ID: https://ampcode.com/threads/T-019c4eef-1568-778f-aede-38254728f848
Co-authored-by: Amp <amp@ampcode.com>

app/assets/database/models.py

@@ -84,6 +84,7 @@ class AssetCacheState(Base):
     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)
+    enrichment_level: Mapped[int] = mapped_column(Integer, nullable=False, default=0)
 
     asset: Mapped[Asset] = relationship(back_populates="cache_states")
 
@@ -91,9 +92,14 @@ class AssetCacheState(Base):
         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"),
+        Index("ix_asset_cache_state_enrichment_level", "enrichment_level"),
         CheckConstraint(
             "(mtime_ns IS NULL) OR (mtime_ns >= 0)", name="ck_acs_mtime_nonneg"
         ),
+        CheckConstraint(
+            "enrichment_level >= 0 AND enrichment_level <= 2",
+            name="ck_acs_enrichment_level_range",
+        ),
         UniqueConstraint("file_path", name="uq_asset_cache_state_file_path"),
     )