Stage 2: DB client factory + live integration tests (real Postgres)

- packages/schema/src/client.ts: createDb(url) -> { db, sql } drizzle/postgres-js
  factory. Exposed as the ./client subpath export so the node-only driver never
  leaks into client bundles.
- packages/schema/src/db.integration.test.ts: 6 tests gated behind INTEGRATION=1
  (content insert/read, vault_path UNIQUE, idempotency_key UNIQUE no-double-post,
  content_id FK, audit append, kill-switch upsert). Verified against Postgres 17.
- Default CI suite unchanged: integration tests skip without INTEGRATION.

Closes the Stage 2.1 DoD against a live database (unblocked by Gate 0.8).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

packages/schema/package.json

@@ -8,6 +8,7 @@
   "exports": {
     ".": "./src/index.ts",
     "./db": "./src/db.ts",
+    "./client": "./src/client.ts",
     "./frontmatter": "./src/frontmatter.ts"
   },
   "scripts": {
@@ -20,7 +21,8 @@
   },
   "dependencies": {
     "zod": "^3.23.0",
-    "drizzle-orm": "^0.36.0"
+    "drizzle-orm": "^0.36.0",
+    "postgres": "^3.4.5"
   },
   "devDependencies": {
     "drizzle-kit": "^0.28.0",