feat(db): add PostgreSQL backend

[cbartz]

Closes #753

What

Add PostgreSQL as a third SQL backend alongside SQLite and MySQL, selectable via
backend = "postgresql":

• config — PostgreSQL backend type, connection settings, validation, and DSN
  construction (sslmode, port-range checks, special-character quoting).
• driver wiring — open the pgx-backed GORM driver, apply connection-pool
  tuning (max open/idle, lifetimes), and reuse the main connection for blob
  storage (PostgreSQL needs no separate objects database like SQLite uses).
• file store — store file-object blobs in PostgreSQL Large Objects
  (keyed by lo_oid), not a bytea column. Large Objects support chunked,
  seekable streaming via the lo API, so blobs are read/written incrementally
  and never fully held in memory — required because GARM's file-object interface
  is streaming (CreateFileObject(..., io.Reader) /
  OpenFileObjectContent(...) io.ReadCloser), and a bytea value would have to
  be materialized in full. Writes io.Copy the reader into the Large Object
  (hashing inline); reads return a ReadCloser over lo.Read. SQLite continues
  to use the Content BLOB column.
• migrations — a dedicated file_object_migrations tracking table with a
  rename shim so existing SQLite installs preserve their file-object migration
  history, plus a functional LOWER(tag) index and the lo_oid column for
  file_object blobs.
• uuid guard — getUserByID pre-validates UUIDs so PostgreSQL's uuid
  column returns ErrNotFound on malformed input instead of a driver error.
• tests + CI — parametrize the SQL suite to run against PostgreSQL when
  GARM_TEST_POSTGRES_DSN is set, and add a PostgreSQL job to the test workflow.

Notes for reviewers

• The bulk of the diff is vendored dependencies (pgx, gorm.io/driver/postgres
  and transitive deps). The hand-written change is ~970 lines; the rest is
  vendor/.
• Case folding differs subtly between backends: SQLite's LOWER() folds
  ASCII only, PostgreSQL's is locale-aware (folds Unicode). For GARM's
  effectively-ASCII identifiers this is a non-issue, but noting it explicitly.

Testing

• Unit/store suite (local, Docker PostgreSQL): ran the SQL store and config
  suites against postgres:16 (Docker, fsync=off) using the same image,
  credentials, and DSN as the CI job — database/sql and config pass with
  -race -tags testing -count=1.
  ok github.com/cloudbase/garm/database/sql (PostgreSQL backend)
  ok github.com/cloudbase/garm/config
• SQLite (unchanged backend): same suite passes in SQLite mode, so both
  backends are covered.
• Lint: make lint (golangci-lint v2.10.1) is clean — 0 issues.
• CI: adds a "Go Tests (PostgreSQL)" job that runs make go-test against a
  postgres:16 service via GARM_TEST_POSTGRES_DSN, exercising
  migrations/AutoMigrate and the Large Object file store on a real PostgreSQL.
• Manual end-to-end: validated a PostgreSQL-backed GARM end-to-end on
  cbartz/garm — created credentials, repo, and a scale set, and confirmed
  runners register with GitHub and execute jobs.

Future work

The nightly integration suite (test/integration, scheduled in
integration-tests.yml) still runs only against SQLite — its config hardcodes
backend = "sqlite3". The PostgreSQL path was validated manually end-to-end (see
Testing); automating that in the nightly suite (add a PG service + template the
[database] config) would be a natural follow-up.

[gabriel-samfira]

I added docker to the CI runners. You should be able to use the service: stanza. If you run into any missing packages, just ping me.

[gabriel-samfira]

Thank you!