Add bearer token authentication for agent API endpoints

[ColinCee]

What's wrong

The agent API endpoints (/review, /implement) are bound to the Tailscale interface (100.100.146.119:8585) and protected by an ALLOWED_ACTORS trust check on triggered_by. However, triggered_by is a plain JSON field with no cryptographic verification — any client on the tailnet can spoof it by sending a crafted POST request.

This is low urgency since the tailnet boundary is already a strong gate, but it's cheap defense-in-depth.

What done looks like

1. A shared bearer token is validated on /review and /implement endpoints via a FastAPI dependency. Requests without a valid Authorization: Bearer <token> header get a 401 response.

2. The token is read from an environment variable (e.g., AGENT_API_KEY) in the agent service. Use Pydantic Settings to validate it's present at startup.

3. /health and /status/* endpoints remain unauthenticated — they're monitoring-only and contain no sensitive data.

4. Tests cover: valid token → 200, invalid token → 401, missing token → 401, health endpoint → 200 without token.

What the agent can't discover

• The token value will be configured in Dokploy environment variables (agent service) and GitHub Actions secrets. The agent doesn't need to set these up — just read the env var.
• Use fastapi.security.HTTPBearer with a FastAPI dependency for clean integration. This gives automatic OpenAPI docs and a standard auth scheme.
• The current ALLOWED_ACTORS check on triggered_by should remain as a second layer — bearer auth validates the caller is a trusted system, actor check validates the triggering user is allowed.

Out of scope (human follow-up)

The GitHub Actions workflow YAML files (.github/workflows/implement.yaml, .github/workflows/code-review.yaml) need to be updated to pass the token as Authorization: Bearer ${{ secrets.AGENT_API_KEY }} in their curl/HTTP calls. The agent cannot modify workflow files — this must be done manually after the Python changes merge.

What must not break

• Existing trust validation (ALLOWED_ACTORS check) must remain — bearer auth is additive
• /health and status endpoints must remain unauthenticated
• Worker containers don't call the API — they run as subprocesses, so no auth changes needed there