feat: add conditional DB vars check to preflight.sh

When GITEA_DB_TYPE is not sqlite3, validate that GITEA_DB_HOST,
GITEA_DB_PORT, GITEA_DB_NAME, GITEA_DB_USER, and GITEA_DB_PASSWD
are set in the required vars check.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

preflight.sh

@@ -260,6 +260,16 @@ check_required_vars() {
     fi
   done
 
+  # DB vars are conditional on GITEA_DB_TYPE (required when NOT sqlite3)
+  if [[ "${GITEA_DB_TYPE:-sqlite3}" != "sqlite3" ]]; then
+    for var in GITEA_DB_HOST GITEA_DB_PORT GITEA_DB_NAME GITEA_DB_USER GITEA_DB_PASSWD; do
+      if [[ -z "${!var:-}" ]]; then
+        log_error "  → Missing required var: $var (required when GITEA_DB_TYPE=${GITEA_DB_TYPE})"
+        missing=1
+      fi
+    done
+  fi
+
   # SSL vars are conditional on SSL_MODE:
   # - letsencrypt => SSL_EMAIL is required
   # - existing    => SSL_CERT_PATH + SSL_KEY_PATH are required