feat(db): shared test suite + migration tool (Phase 4 & 5)

[ajianaz]

Summary

Completes the Multi-Backend Database epic (#171) — Phase 4 and Phase 5.

Closes #169, closes #170.

Phase 4: Shared Test Suite (#169)

Backend-agnostic tests that run identically against SQLite and Postgres:

Test	Operations
project_crud	Create, read (slug/id/dsn), update, rotate, archive, delete
issue_upsert_dedup	Fingerprint dedup, count increment, status, list filtered
event_operations	Insert, list, pagination, raw fetch
auth_and_sessions	User CRUD, session lifecycle, expiry cleanup
auth_attempts	Brute-force tracking, email + IP counting
alert_rules	CRUD, toggle, history, cooldown
search	LIKE/ILIKE, level filter, count
retention	Purge old events, orphan cleanup
count_table	Whitelist validation
ping	Health check

• SQLite: 11/11 tests pass (in-memory)
• Postgres: compiles clean, gated behind TEST_POSTGRES_URL

Phase 5: Migration Tool (#170)

# Export SQLite → JSONL
trapfall db export --from sqlite:trapfall.db --to dump.jsonl

# Import JSONL → Postgres
trapfall db import --from dump.jsonl --to postgres://user@host/db

# Verify
trapfall db verify --url postgres://user@host/db

• trapfalld/src/migrate.rs — export/import/verify logic
• CLI: Db { Export, Import, Verify } subcommands
• docs/guide/migration.md — step-by-step guide

Test Plan

• cargo fmt --check — clean
• cargo clippy --all-targets -- -D warnings — zero warnings
• cargo test --workspace — all pass
• SQLite shared tests: 11/11 pass
• Postgres shared tests: compile clean (need live DB to run)

Epic #171 Status

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