perf(db): drop redundant idx_edges_source / idx_edges_target (#142)

Both narrow indexes are fully covered by the existing (source, kind)
and (target, kind) composites via SQLite's left-prefix scan, so
they're dead weight on every write. Empirical measurements (from the
spike script in PR #122 on a 50K-node / 250K-edge synthetic DB):

  - DB size: 34.7 MB → 27.0 MB (-22.2%)
  - Bulk insert (250K edges): 590ms → 431ms (1.37× faster)
  - source/target lookup latency: no regression

Adds migration v4 to drop both on existing databases; fresh-DB schema
no longer creates them.

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/db/migrations.ts

@@ -9,7 +9,7 @@ import { SqliteDatabase } from './sqlite-adapter';
 /**
  * Current schema version
  */
-export const CURRENT_SCHEMA_VERSION = 3;
+export const CURRENT_SCHEMA_VERSION = 4;
 
 /**
  * Migration definition
@@ -54,6 +54,17 @@ const migrations: Migration[] = [
       `);
     },
   },
+  {
+    version: 4,
+    description:
+      'Drop redundant idx_edges_source / idx_edges_target (covered by source_kind / target_kind composites)',
+    up: (db) => {
+      db.exec(`
+        DROP INDEX IF EXISTS idx_edges_source;
+        DROP INDEX IF EXISTS idx_edges_target;
+      `);
+    },
+  },
 ];
 
 /**