refactor(backend): restructure into infrastructure/transport layers, add ENV-driven logger

[GRACENOBLE]

Summary

• New pkg/logger — New(env string) returns a JSON slog handler for staging/production and text for everything else; ENV env var controls the format
• Bootstrap wires the logger — slog.SetDefault is called immediately so all slog.* calls across the process share the configured handler; Gin runs in release mode for staging/production suppressing [GIN-debug] output
• internal/handler/ → internal/transport/handlers/ and new internal/transport/middleware/ with a structured slog Gin access logger replacing Gin's built-in unstructured logger
• internal/repository/postgres/ → internal/infrastructure/database/postgres/ — DB implementations live under infrastructure
• backend/migrations/ → internal/infrastructure/database/migrations/ — migrations are internal to the project, cmd/migrate migrationsDir constant updated accordingly
• CLAUDE.md and all backend/docs/ updated to reflect new paths and conventions

Test plan

• ENV=staging make watch — all logs including Gin access logs are JSON, no [GIN-debug] lines
• ENV=local make watch — logs are human-readable text, [GIN-debug] route list visible
• make migrate-status — goose finds migrations at new path
• make migrate-create name=test_slug — new file created under internal/infrastructure/database/migrations/
• go build ./... and go vet ./... pass
• make test passes (unit tests; integration requires Docker)

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: e77f6e57-2fc6-4d85-882b-6c1b3fdf04ae

📥 Commits

Reviewing files that changed from the base of the PR and between 9e7bfe7 and 0f42ff8.

📒 Files selected for processing (26)

• CLAUDE.md
• backend/Makefile
• backend/cmd/api/main.go
• backend/cmd/migrate/main.go
• backend/docs/_index.md
• backend/docs/bootstrap.md
• backend/docs/database.md
• backend/docs/environment.md
• backend/docs/error-handling.md
• backend/docs/migrations.md
• backend/docs/routing.md
• backend/docs/testing.md
• backend/internal/bootstrap/bootstrap.go
• backend/internal/infrastructure/database/migrations/.gitkeep
• backend/internal/infrastructure/database/migrations/20260614201325_init.sql
• backend/internal/infrastructure/database/postgres/db.go
• backend/internal/infrastructure/database/postgres/health_repository.go
• backend/internal/infrastructure/database/postgres/health_repository_test.go
• backend/internal/server/server.go
• backend/internal/transport/handlers/handler.go
• backend/internal/transport/handlers/health_handler.go
• backend/internal/transport/handlers/hello_handler.go
• backend/internal/transport/handlers/hello_handler_test.go
• backend/internal/transport/handlers/routes.go
• backend/internal/transport/middleware/logger.go
• backend/pkg/logger/logger.go

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch backend.environments-and-logger

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