fix. harden Go review flow

Fix runner bootstrap and auth handling, preserve queued SHAs, make event/job acceptance atomic, fence stale runs, correct retries and prompts, add fake end-to-end coverage, and fix deployment defaults.

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

.env.example

@@ -41,6 +41,7 @@ DB_PORT=3306
 DB_NAME=gitea_codex
 DB_USER=gitea_codex
 DB_PASSWORD=replace
+MARIADB_ROOT_PASSWORD=replace-with-a-different-root-password
 
 WORKDIR=/var/lib/gitea-codex/worktrees
 MAX_DIFF_BYTES=200000
@@ -53,7 +54,7 @@ REVIEW_RUNNER_IMAGE=node:22-bookworm-slim
 # Security: fork PRs are skipped unless explicitly enabled.
 ALLOW_UNTRUSTED_FORKS=false
 
-# Optional SQLite/MariaDB override. When unset, DB_* values compose a MariaDB DSN.
-DATABASE_URL=sqlite://./gitea-codex.db
+# Optional database override. Leave blank for the MariaDB DSN composed from DB_*.
+DATABASE_URL=
 WEBHOOK_MAX_BYTES=2097152
 PORT=8000