Add content-addressable runs and activities

- compute_run_id() for deterministic run identification
- /assets/by-run-id/{run_id} endpoint for L1 recovery
- Store run_id in provenance when recording runs
- Activities now use content hash as ID instead of UUID
- URLs: /activities/{content_hash} instead of /activities/1

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

db.py

@@ -65,9 +65,9 @@ CREATE TABLE IF NOT EXISTS assets (
     updated_at TIMESTAMPTZ
 );
 
--- Activities table
+-- Activities table (activity_id is content-addressable run_id hash)
 CREATE TABLE IF NOT EXISTS activities (
-    activity_id UUID PRIMARY KEY,
+    activity_id VARCHAR(64) PRIMARY KEY,
     activity_type VARCHAR(50) NOT NULL,
     actor_id TEXT NOT NULL,
     object_data JSONB NOT NULL,
@@ -83,8 +83,8 @@ CREATE TABLE IF NOT EXISTS anchors (
     tree_ipfs_cid VARCHAR(128),
     ots_proof_cid VARCHAR(128),
     activity_count INTEGER NOT NULL,
-    first_activity_id UUID,
-    last_activity_id UUID,
+    first_activity_id VARCHAR(64),
+    last_activity_id VARCHAR(64),
     created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     confirmed_at TIMESTAMPTZ,
     bitcoin_txid VARCHAR(64)
@@ -243,6 +243,20 @@ async def get_asset_by_hash(content_hash: str) -> Optional[dict]:
         return None
 
 
+async def get_asset_by_run_id(run_id: str) -> Optional[dict]:
+    """Get asset by run_id stored in provenance."""
+    async with get_connection() as conn:
+        row = await conn.fetchrow(
+            """SELECT name, content_hash, ipfs_cid, asset_type, tags, metadata, url,
+                      provenance, description, origin, owner, created_at, updated_at
+               FROM assets WHERE provenance->>'run_id' = $1""",
+            run_id
+        )
+        if row:
+            return _parse_asset_row(row)
+        return None
+
+
 async def get_all_assets() -> dict[str, dict]:
     """Get all assets as a dict indexed by name."""
     async with get_connection() as conn: