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).

packages/import/package.json

@@ -7,14 +7,17 @@
   "scripts": {
     "start": "tsx --env-file=../../.env.local src/index.ts",
     "import-hibp": "tsx --env-file=../../.env.local src/importHibp.ts",
+    "bulk-import": "tsx --env-file=../../.env.local src/bulkImport.ts",
     "build": "tsc -p tsconfig.json",
     "test": "echo \"Error: no test specified\" && exit 1"
   },
   "license": "ISC",
   "dependencies": {
-    "pg": "^8.22.0"
+    "pg": "^8.22.0",
+    "pg-copy-streams": "^7.0.0"
   },
   "devDependencies": {
-    "@types/pg": "^8.20.0"
+    "@types/pg": "^8.20.0",
+    "@types/pg-copy-streams": "^1.2.5"
   }
 }