✨ feat(bdd): parallel-safe schema-per-package isolation (T12 stage 2/2)

Re-enables BDD_SCHEMA_ISOLATION=true with the foundation from PR #34
(NewPostgresRepositoryFromDSN). Achieves ~2.85x speedup on the BDD
test suite by running feature packages in parallel.

ARCHITECTURE

When BDD_SCHEMA_ISOLATION=true, each test package (process) gets its
own isolated PostgreSQL schema:
  1. testserver.Start() generates a deterministic schema name per FEATURE
  2. CREATE SCHEMA <name>
  3. Open a per-package gorm.DB with DSN search_path=<name>
  4. AutoMigrate runs in the isolated schema (creates users table)
  5. Build a per-package server.Server with this isolated repo via
     server.NewServerWithUserRepo
  6. Stop() drops the schema + closes the per-package pool

Packages then run in parallel (default Go test parallelism) without
contention because each has its own schema + connection pool.

CHANGES

- pkg/server/server.go : NEW factory NewServerWithUserRepo(cfg, ctx,
  userRepo, userService) that injects a per-test repo. Existing NewServer
  becomes a thin wrapper.
- pkg/bdd/testserver/server.go : Start() chooses isolated mode based on
  BDD_SCHEMA_ISOLATION env var. Stop() drops schema + closes pool.
- pkg/user/postgres_repository.go : Exec(sql) helper for the schema
  lifecycle (CREATE/DROP) used by testserver.
- scripts/run-bdd-tests.sh : -p 1 only when BDD_SCHEMA_ISOLATION!=true.
  When true, default Go parallelism (~ NumCPU packages concurrent).
- .gitea/workflows/ci-cd.yaml : exports BDD_SCHEMA_ISOLATION=true.
- adr/0025-bdd-scenario-isolation-strategies.md : Status to "Implemented".

VALIDATION

5x AuthBDD with isolation: 5/5 PASS, public.users count=0 after runs.

Local benchmark on the full features/... suite:
- Sequential -p 1 (no isolation):     12.87s
- Parallel + isolation (this PR):      4.51s
- Speedup: 2.85x

🤖 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pkg/user/postgres_repository.go

@@ -184,7 +184,15 @@ func BuildSchemaIsolatedDSN(cfg *config.Config, schemaName string) string {
 	)
 }
 
-// (Close already exists below; we reuse it.)
+// Exec runs a raw SQL statement against the repository's connection.
+// Used by BDD test infra for schema lifecycle (CREATE SCHEMA / DROP SCHEMA).
+// Avoid in production code paths -- prefer the typed Repository methods.
+func (r *PostgresRepository) Exec(sql string) error {
+	if r.db == nil {
+		return fmt.Errorf("Exec called on PostgresRepository with nil db")
+	}
+	return r.db.Exec(sql).Error
+}
 
 // initializeDatabase sets up the PostgreSQL database connection and runs migrations
 func (r *PostgresRepository) initializeDatabase() error {