test: add comprehensive test suite for backend

Add 56 tests covering the core backend packages:

Unit tests (no database required):
- internal/partnum: 7 tests for part number generation logic
  (sequence, format templates, enum validation, constants)
- internal/schema: 8 tests for YAML schema loading, property
  merging, validation, and default application

Integration tests (require TEST_DATABASE_URL):
- internal/db/items: 10 tests for item CRUD, archive/unarchive,
  revisions, and thumbnail operations
- internal/db/relationships: 10 tests for BOM CRUD, cycle detection,
  self-reference blocking, where-used, expanded/flat BOM
- internal/db/projects: 5 tests for project CRUD and item association
- internal/api/bom_handlers: 6 HTTP handler tests for BOM endpoints
  including flat BOM, cost calculation, add/delete entries
- internal/api/items: 5 HTTP handler tests for item CRUD endpoints

Infrastructure:
- internal/testutil: shared helpers for test DB pool setup,
  migration runner, and table truncation
- internal/db/helpers_test.go: DB wrapper for integration tests
- internal/db/db.go: add NewFromPool constructor
- Makefile: add test-integration target with default DSN

Integration tests skip gracefully when TEST_DATABASE_URL is unset.
Dev-mode auth (nil authConfig) used for API handler tests.

Fixes: fmt.Errorf Go vet warning in partnum/generator.go

Closes #2

Makefile

@@ -1,4 +1,4 @@
-.PHONY: build run test clean migrate fmt lint \
+.PHONY: build run test test-integration clean migrate fmt lint \
         docker-build docker-up docker-down docker-logs docker-ps \
         docker-clean docker-rebuild \
         web-install web-dev web-build
@@ -7,8 +7,8 @@
 # Local Development
 # =============================================================================
 
-# Build all binaries
-build:
+# Build all binaries (frontend + backend)
+build: web-build
 	go build -o silo ./cmd/silo
 	go build -o silod ./cmd/silod
 
@@ -20,14 +20,19 @@ run:
 cli:
 	go run ./cmd/silo $(ARGS)
 
-# Run tests
+# Run unit tests (integration tests skipped without TEST_DATABASE_URL)
 test:
 	go test -v ./...
 
+# Run all tests including integration tests (requires PostgreSQL)
+test-integration:
+	TEST_DATABASE_URL="postgres://silo:silodev@localhost:5432/silo_test?sslmode=disable" go test -v -count=1 ./...
+
 # Clean build artifacts
 clean:
 	rm -f silo silod
 	rm -f *.out
+	rm -rf web/dist
 
 # Format code
 fmt:
@@ -153,7 +158,8 @@ help:
 	@echo "  build          - Build CLI and server binaries"
 	@echo "  run            - Run API server locally"
 	@echo "  cli ARGS=...   - Run CLI with arguments"
-	@echo "  test           - Run tests"
+	@echo "  test           - Run unit tests (integration tests skip without DB)"
+	@echo "  test-integration - Run all tests including integration (needs PostgreSQL)"
 	@echo "  fmt            - Format code"
 	@echo "  lint           - Run linter"
 	@echo "  tidy           - Tidy go.mod"