✨ 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>

scripts/run-bdd-tests.sh

@@ -144,9 +144,21 @@ run_tests_with_tags() {
         # Note: -tags flag in go test is for Go build tags, NOT Godog feature tags
         # We use GODOG_TAGS env var which is read by the test framework
         echo "🚀 Running: GODOG_TAGS=\"${DEFAULT_TAGS}\" go test ./features/..."
-        # -p 1 forces sequential package execution to avoid DB-state contention between feature packages
-        # (different packages would otherwise spawn concurrent test servers sharing the same Postgres DB).
-        GODOG_TAGS="$DEFAULT_TAGS" go test ./features/... -v -p 1 -cover -coverpkg=./... -coverprofile=coverage.out 2>&1 | tee /tmp/bdd_test_output.txt && test_output=$(cat /tmp/bdd_test_output.txt) && rm -f /tmp/bdd_test_output.txt || test_output=$(cat /tmp/bdd_test_output.txt 2>/dev/null || echo "")
+        # When BDD_SCHEMA_ISOLATION=true (T12 architecture):
+        #   each test PACKAGE gets its own isolated PostgreSQL schema with its own
+        #   connection pool + migrations (cf. pkg/bdd/testserver/server.go Start()).
+        #   Packages then run in parallel safely. ~2.85x speedup observed locally.
+        # When unset:
+        #   fall back to -p 1 (sequential). Uses public schema with TRUNCATE-style
+        #   cleanup between scenarios.
+        if [ "${BDD_SCHEMA_ISOLATION:-}" = "true" ]; then
+          PARALLEL_FLAG=""
+          echo "🔀 BDD_SCHEMA_ISOLATION=true → feature packages run in parallel"
+        else
+          PARALLEL_FLAG="-p 1"
+          echo "🐌 BDD_SCHEMA_ISOLATION not set → feature packages run sequentially (-p 1)"
+        fi
+        GODOG_TAGS="$DEFAULT_TAGS" go test ./features/... -v $PARALLEL_FLAG -cover -coverpkg=./... -coverprofile=coverage.out 2>&1 | tee /tmp/bdd_test_output.txt && test_output=$(cat /tmp/bdd_test_output.txt) && rm -f /tmp/bdd_test_output.txt || test_output=$(cat /tmp/bdd_test_output.txt 2>/dev/null || echo "")
         test_exit_code=${PIPESTATUS[0]}
     fi