fix: add ON DELETE CASCADE to uploads FK on snapshot_id

The uploads table's foreign key on snapshot_id did not cascade deletes,
unlike snapshot_files and snapshot_blobs. This caused FK violations when
deleting snapshots with associated upload records (if FK enforcement is
enabled) unless uploads were manually deleted first.

closes #18

internal/database/schema.sql

@@ -130,7 +130,7 @@ CREATE TABLE IF NOT EXISTS uploads (
     size INTEGER NOT NULL,
     duration_ms INTEGER NOT NULL,
     FOREIGN KEY (blob_hash) REFERENCES blobs(blob_hash),
-    FOREIGN KEY (snapshot_id) REFERENCES snapshots(id)
+    FOREIGN KEY (snapshot_id) REFERENCES snapshots(id) ON DELETE CASCADE
 );
 
 -- Index for efficient snapshot lookups