fix: pin database-postgres driver to the golden tuple (pg 8.22.0, Kysely 0.29.4)

Review finding on PR #391: packages/database-postgres pinned pg@8.23.0 and
kysely@0.29.5, but the architecture doc's golden tuple (Technology-Stack
section 5.2 / section 7) pins pg@8.22.0 and Kysely@0.29.4. Reproducibility
requires the exact documented versions.

- packages/database-postgres/package.json: pg 8.23.0 -> 8.22.0,
  kysely 0.29.5 -> 0.29.4, @types/pg 8.23.1 -> 8.21.0 (no 8.22.x of
  @types/pg is published; 8.21.0 is the closest matching release, types
  for the immediately preceding pg minor)
- pnpm-lock.yaml: regenerated with pnpm 11.23.0 (Node 24); the resolved
  pg dependency tree is unchanged apart from the driver version itself
- tests/database-postgres-imports.test.mjs: exact-pin assertions updated
  to the corrected versions, with a comment noting the @types/pg choice

packages/database-postgres/package.json

@@ -9,11 +9,11 @@
     "typecheck": "tsc -p tsconfig.json --noEmit"
   },
   "dependencies": {
-    "kysely": "0.29.5",
-    "pg": "8.23.0"
+    "kysely": "0.29.4",
+    "pg": "8.22.0"
   },
   "devDependencies": {
-    "@types/pg": "8.23.1"
+    "@types/pg": "8.21.0"
   },
   "main": "./dist/index.js",
   "types": "./dist/index.d.ts",