feat(db): connection factory + env config + feature flags (Phase 2)

[ajianaz]

Summary

Implements Phase 2 of the Multi-Backend Database epic (#171).

Adds a connection factory that detects the database URL scheme and instantiates the correct backend. Users select a backend via TRAPFALL_DATABASE_URL env var or --db flag.

Closes #167.

Changes

• trapfall_db::open_database(url) — factory function detecting sqlite: / postgres: URL schemes
• trapfall_db::normalise_url(url) — bare paths (e.g. trapfall.db) auto-prefixed with sqlite:
• Cargo feature flags: default = ["sqlite"], optional postgres
  • sqlite module gated behind #[cfg(feature = "sqlite")]
  • Root sqlx dep trimmed to runtime-tokio; each crate opts-in its backend
• TRAPFALL_DATABASE_URL env var support via clap env feature
  • --db changed from PathBuf to String (accepts URLs)
  • Fallback: --db flag → TRAPFALL_DATABASE_URL env → trapfall.db default
• Docker: TRAPFALL_DATABASE_URL env var in docker-compose.yml
  • Dockerfile: cache trapfall-db/Cargo.toml in dependency stage
• 9 factory tests: URL detection, feature flags, error messages, trait object sanity

Acceptance Criteria

Criteria	Status
TRAPFALL_DATABASE_URL=sqlite:./trapfall.db works	✅
postgres://... errors clearly if postgres feature not enabled	✅
cargo build (default) = SQLite only	✅
cargo build --features postgres = both backends compiled	✅
Docker container starts with correct backend	✅

Test Plan

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — zero warnings
• cargo test --workspace — 93 tests passed, 0 failed
• cargo build --release -p trapfalld — binary builds
• trapfall --help — shows database URL documentation
• TRAPFALL_DATABASE_URL=sqlite:/tmp/test.db trapfall healthcheck — exit 0
• trapfall --db /tmp/test.db healthcheck — exit 0 (bare path)
• trapfall --db postgres://localhost/db healthcheck — clear error, exit 1

Roadmap

Phase	Issue	Status
1 — Trait abstraction	#186	✅ Merged
2 — Connection factory	#167	This PR
3 — Postgres backend	#168	Next (v0.1.0)
4 — Shared test suite	#169	Next (v0.1.0)
5 — Migration tool	#170	Next (v0.1.0)