Add fail-closed author allowlist gating job dispatch

Sessions run claude with --dangerously-skip-permissions and a prompt built
from issue/PR/comment text, so only trusted authors may supply that text.
teabot now dispatches a job only when the triggering event's author login is
on an allowlist; an empty allowlist dispatches nothing (fail-closed).

Claude-Session: https://claude.ai/code/session_015ur3i7D2azsMAWTSVABApv

docs/configuration.md

@@ -21,6 +21,8 @@ Override the path with `--config/-c`. Generate a starting point with
 | `anthropic_api_key` | *(unset)* | If set, injected as `ANTHROPIC_API_KEY`. |
 | `anthropic_base_url` | *(unset)* | If set, injected as `ANTHROPIC_BASE_URL`. |
 | `state_dir` | `~/.local/state/teabot` | Where processed-event state is persisted. |
+| `allowed_authors` | *(empty → fail-closed)* | Gitea usernames whose events may trigger a job. See [Author allowlist](#author-allowlist-security). |
+| `repo_allowed_authors` | *(unset)* | Optional per-repo override of `allowed_authors`, keyed by `owner/name`. |
 | `personalities` | *(required)* | Bot identities (see below). |
 
 `~` and `~/` are expanded in path-valued keys.
@@ -73,6 +75,45 @@ prevention** (it never reacts to events authored by any personality's username).
 At least one personality must be able to implement and at least one to review,
 or config validation fails.
 
+## Author allowlist (security)
+
+Each job runs `claude --print --dangerously-skip-permissions` inside a container,
+and its **prompt is built from event text** — the issue title/body, PR
+description/diff, and comments. That text is attacker-controllable: anyone who
+can open an issue or comment on the repo could otherwise inject instructions
+into a session that has permissions disabled. The allowlist is the control that
+closes this: teabot dispatches a job **only** when the triggering event's author
+login is on the allowlist.
+
+```yaml
+# Global allowlist.
+allowed_authors:
+  - benvin
+  - trusted-colleague
+
+# Optional per-repo override. A present entry fully REPLACES the global list for
+# that repo; an empty list disables dispatch for it; an absent entry falls back
+# to the global list.
+repo_allowed_authors:
+  unkin/teabot:
+    - benvin
+```
+
+Semantics:
+
+- **Fail-closed.** An empty/absent allowlist dispatches **nothing**. teabot logs
+  a warning at startup when no allowlist is configured anywhere.
+- Events from non-allowlisted authors are **recorded** as processed (so they
+  don't re-trigger) and logged at info with the author name, but never spawn a
+  container. They are *not* marked as engaged threads.
+- **Comment follow-ups are gated on the new comment's author**, not just the
+  thread. A comment from a non-allowlisted user on a thread teabot previously
+  acted on is ignored — otherwise an untrusted comment could reopen the
+  injection path on a bot thread.
+- Matching is case-insensitive (Gitea usernames are unique case-insensitively).
+- This is **separate from and in addition to** loop prevention: bot personality
+  logins are always excluded first, even if one were listed here.
+
 ## Claude credentials
 
 By default teabot uses your Claude **subscription** auth: it copies