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>

alembic_db/versions/0003_add_enrichment_level.py

@@ -0,0 +1,44 @@
+"""
+Add enrichment_level column to asset_cache_state for phased scanning
+
+Level 0: Stub record (path, size, mtime only)
+Level 1: Metadata extracted (safetensors header, mime type)
+Level 2: Hash computed (blake3)
+
+Revision ID: 0003_add_enrichment_level
+Revises: 0002_add_is_missing
+Create Date: 2025-02-10 00:00:00
+"""
+
+from alembic import op
+import sqlalchemy as sa
+
+revision = "0003_add_enrichment_level"
+down_revision = "0002_add_is_missing"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "asset_cache_state",
+        sa.Column(
+            "enrichment_level",
+            sa.Integer(),
+            nullable=False,
+            server_default=sa.text("0"),
+        ),
+    )
+    op.create_index(
+        "ix_asset_cache_state_enrichment_level",
+        "asset_cache_state",
+        ["enrichment_level"],
+    )
+    # Treat existing records as fully enriched (level 1 = metadata done)
+    # since they were created with the old scanner that extracted metadata
+    op.execute("UPDATE asset_cache_state SET enrichment_level = 1")
+
+
+def downgrade() -> None:
+    op.drop_index("ix_asset_cache_state_enrichment_level", table_name="asset_cache_state")
+    op.drop_column("asset_cache_state", "enrichment_level")