feat: self-generate and store the terraform registry signing key

Rather than requiring an operator to create a GPG key and a K8s secret, the
registry now provisions itself: on first start artifactapi generates a signing
keypair and persists it in a new signing_keys table, so all replicas share one
key and there is nothing to set up. TF_SIGNING_KEY_PATH still overrides with a
bring-your-own key when set.

- signing_keys table + GetSigningKey / InsertSigningKeyIfAbsent (ON CONFLICT DO
  NOTHING so a replica race converges on one key)
- tfsign.Generate, LoadArmored, and LoadOrCreate(store, purpose)
- server prefers a configured key file, else LoadOrCreate against the DB
- tests: generate/load round-trip, load-or-create generates once then reuses,
  DB insert idempotency

internal/database/postgres.go

@@ -158,6 +158,13 @@ func (db *DB) migrate() error {
 		);
 
 		CREATE INDEX IF NOT EXISTS idx_rpm_metadata_repo ON rpm_metadata(repo_name);
+
+		CREATE TABLE IF NOT EXISTS signing_keys (
+			purpose             TEXT PRIMARY KEY,
+			private_key_armor   TEXT NOT NULL,
+			key_id              TEXT NOT NULL,
+			created_at          TIMESTAMPTZ DEFAULT NOW()
+		);
 	`)
 	return err
 }