feat: add database-postgres driver boundary package (E00-S03-T02)

- packages/database-postgres (@personal-blog/database-postgres): the single
  workspace package allowed to import the PostgreSQL driver — declares pg and
  kysely as exact dependencies (@types/pg for types) and its src/index.ts
  imports and re-exports the driver pieces (Pool, Kysely, PostgresDialect) so
  the isolation is real, not a placeholder
- pnpm-lock.yaml: importer for packages/database-postgres plus the resolved
  pg/kysely dependency tree (frozen-lockfile install keeps working)
- .gitea/workflows/ci.yml: new database-postgres-imports job runs
  tests/database-postgres-imports.test.mjs on every PR so the isolation
  criterion gates merges

packages/database-postgres/src/index.ts

@@ -0,0 +1,20 @@
+/**
+ * @personal-blog/database-postgres — EPPP PostgreSQL adapter package.
+ *
+ * [E00-S03-T02] this package is the single owner of the PostgreSQL driver
+ * stack: it is the only workspace package allowed to import `pg` and Kysely.
+ * Every piece of the workspace that talks to the database goes through this
+ * boundary — the pool, the query builder dialect and the adapter API this
+ * package will expose — so no other package ever imports the driver directly.
+ *
+ * This module is the driver boundary: it imports the PostgreSQL driver (`pg`)
+ * and Kysely and re-exports the pieces later stories build the adapter on
+ * (migration ledger E00-S03-T03, advisory lock E00-S03-T04). Until then the
+ * re-exports keep the driver reachable only from here — the isolation is real,
+ * not a placeholder.
+ */
+
+import { Pool } from 'pg';
+import { Kysely, PostgresDialect } from 'kysely';
+
+export { Pool, Kysely, PostgresDialect };