add systemd service unit and rate limit headers

Systemd deployment:
- examples/flaskpaste.service with security hardening
- examples/flaskpaste.env with all config options
- README deployment section updated

Rate limit headers (X-RateLimit-*):
- Limit, Remaining, Reset on 201 and 429 responses
- Per-IP tracking with auth multiplier
- api.md documented

TASKLIST.md

@@ -10,13 +10,6 @@ Prioritized, actionable tasks. Each task is small and completable in one session
 |--------|--------------------------------------------------------------
 | ☐      | Create Ansible deployment role
 | ☐      | Add Kubernetes manifests (Deployment, Service, ConfigMap)
-| ☐      | Add systemd service unit example
-
-## Priority 2: Features
-
-| Status | Task
-|--------|--------------------------------------------------------------
-| ☐      | Add rate limit headers (X-RateLimit-*)
 
 ## Priority 3: Quality
 
@@ -35,6 +28,8 @@ Prioritized, actionable tasks. Each task is small and completable in one session
 
 | Date       | Task
 |------------|--------------------------------------------------------------
+| 2024-12    | Add systemd service unit example
+| 2024-12    | Add rate limit headers (X-RateLimit-*)
 | 2024-12    | Integrate PKI audit logging (CERT_ISSUED, CERT_REVOKED, AUTH_FAILURE)
 | 2024-12    | Integrate request duration metrics (Prometheus histogram)
 | 2024-12    | Add memory leak detection tests (tracemalloc)