test(infrastructure): Configure pgxpool with max_conns=1

Fix database connection exhaustion in tests by setting max_conns=1
when creating pgxpool via pgxpool.ParseConfig().

- Update setupTestServer() in test_helpers.go
- Update setupSyncTestDB() in sync_integration_test.go

This reduces per-test connection usage from 4 to 1, keeping total
connections well under PostgreSQL's default max_connections=100.

78 tests × 1 connection = 78 connections (down from 312 potential)

Fixes test failures: "FATAL: sorry, too many clients already"

See PROJECT_GUIDELINES.md Testing section for details.

PROJECT_GUIDELINES.md

@@ -9,9 +9,19 @@
 - ❌ **NEVER build server binaries locally** - all builds through Dockerfile/docker-compose
 - ❌ **NEVER create new migration files** - merge changes into current one until release
 - ❌ **NEVER use `git checkout` on schema files** without checking what will be lost
-- ❌ **NEVER break existing functionality** unless explicitly instructed
-- ❌ **NEVER duplicate business logic** - keep logic in services, not handlers
-- ❌ **NEVER bypass service layer** - all database operations must go through services
+ - ❌ **NEVER break existing functionality** unless explicitly instructed
+ - ❌ **NEVER duplicate business logic** - keep logic in services, not handlers
+ - ❌ **NEVER bypass service layer** - all database operations must go through services
+ 
+### Testing
+- ✅ **ALWAYS use `setupTestServer()` helper from `cmd/server/tests/test_helpers.go`**
+- ✅ **Share one test setup across all subtests** - call `setupTestServer()` once at test function level, not per subtest
+- ✅ **Prefer table-driven tests** - use `t.Run()` with test cases instead of duplicate test functions
+- ✅ **Configure database pools efficiently** - use `max_conns=1` for test pools (via `pgxpool.ParseConfig()`) to prevent connection exhaustion
+- ❌ **NEVER create separate `pgxpool` per test** - each pool creates 4 connections by default; 78 tests = 312 potential connections > PostgreSQL's 100 limit
+- ❌ **NEVER call `setupTestServer()` in loops or within subtests** - creates unnecessary database pools and exhausts connections
+- ✅ **DO verify tests pass** - run full test suite before completing work
+- ✅ **Use `t.Cleanup()` properly** - the `TestServerSetup` pattern automatically handles cleanup via `t.Cleanup()`
 
 ### Frontend & Styling
 - ❌ **NEVER modify backend/API for frontend features without user confirmation**