schema: add ON DELETE CASCADE to snapshot_files.file_id and snapshot_blobs.blob_id FKs (#46)

Add `ON DELETE CASCADE` to the two foreign keys that were missing it:

- `snapshot_files.file_id` → `files(id)`
- `snapshot_blobs.blob_id` → `blobs(id)`

This ensures that when a file or blob row is deleted, the corresponding snapshot junction rows are automatically cleaned up, consistent with the other CASCADE FKs already in the schema.

closes #19

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #46
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

internal/database/schema.sql

@@ -103,7 +103,7 @@ CREATE TABLE IF NOT EXISTS snapshot_files (
     file_id TEXT NOT NULL,
     PRIMARY KEY (snapshot_id, file_id),
     FOREIGN KEY (snapshot_id) REFERENCES snapshots(id) ON DELETE CASCADE,
-    FOREIGN KEY (file_id) REFERENCES files(id)
+    FOREIGN KEY (file_id) REFERENCES files(id) ON DELETE CASCADE
 );
 
 -- Index for efficient file lookups (used in orphan detection)
@@ -116,7 +116,7 @@ CREATE TABLE IF NOT EXISTS snapshot_blobs (
     blob_hash TEXT NOT NULL,
     PRIMARY KEY (snapshot_id, blob_id),
     FOREIGN KEY (snapshot_id) REFERENCES snapshots(id) ON DELETE CASCADE,
-    FOREIGN KEY (blob_id) REFERENCES blobs(id)
+    FOREIGN KEY (blob_id) REFERENCES blobs(id) ON DELETE CASCADE
 );
 
 -- Index for efficient blob lookups (used in orphan detection)