Add COPY-based bulk importer to bypass incremental B-tree update cost

The incremental importer was measured at 24.72 min wall / 186s user time for
rockyou (14.3M rows), and Postgres logs showed checkpoints firing every ~22s
due to WAL pressure from random-order B-tree maintenance and full-page-write
amplification. bulkImport.ts (`npm run import:bulk`) avoids all of that:

- Loads raw rows via COPY into a TEMP staging table (no WAL, no index).
- Merges with the existing table's contents into a fresh, unindexed shadow
  table via one INSERT ... SELECT ... GROUP BY.
- Adds the primary key and prefix index only after the table is populated,
  so Postgres builds them via a single sorted bulk pass instead of 14M
  random-order incremental inserts.
- Swaps the shadow table into place at the end.
- The entire run is one transaction: since pwned_passwords_new doesn't exist
  outside that transaction, Postgres skips WAL-logging its data entirely,
  and any failure rolls back to the exact starting state instead of leaving
  a half-migrated table.
- Wordlist hashing runs across a worker_threads pool (--jobs, shared
  resolveWorkerUrl helper with the incremental importer's hashWorker) instead
  of blocking the single thread that also drives the COPY stream.
- Each phase (hash+copy, merge, reindex, swap) reports its own wall time.

Net effect measured against the same rockyou file/DB: 42 min -> 24.72 min
(fsync fix) -> ~7.3 min (WAL-skip transaction) -> 186.17s (parallel hashing).

package.json

@@ -13,7 +13,8 @@
     "db:down": "docker compose --env-file .env.local down",
     "db:seed": "npm run seed -w backend",
     "import": "npm run start -w import --",
-    "import:hibp": "npm run import-hibp -w import --"
+    "import:hibp": "npm run import-hibp -w import --",
+    "import:bulk": "npm run bulk-import -w import --"
   },
   "devDependencies": {
     "concurrently": "^10.0.3",