feat: add observability and CLI enhancements

Audit logging:
- audit_log table with event tracking
- app/audit.py module with log_event(), query_audit_log()
- GET /audit endpoint (admin only)
- configurable retention and cleanup

Prometheus metrics:
- app/metrics.py with custom counters
- paste create/access/delete, rate limit, PoW, dedup metrics
- instrumentation in API routes

CLI clipboard integration:
- fpaste create -C/--clipboard (read from clipboard)
- fpaste create --copy-url (copy result URL)
- fpaste get -c/--copy (copy content)
- cross-platform: xclip, xsel, pbcopy, wl-copy

Shell completions:
- completions/ directory with bash/zsh/fish scripts
- fpaste completion --shell command

app/api/__init__.py

@@ -13,11 +13,13 @@ _cleanup_times: dict[str, float] = {
     "pastes": 0.0,
     "hashes": 0.0,
     "rate_limits": 0.0,
+    "audit": 0.0,
 }
 _CLEANUP_INTERVALS = {
     "pastes": 3600,  # 1 hour
     "hashes": 900,  # 15 minutes
     "rate_limits": 300,  # 5 minutes
+    "audit": 86400,  # 24 hours
 }
 
 
@@ -61,5 +63,15 @@ def run_scheduled_cleanup():
             if count > 0:
                 current_app.logger.info(f"Cleaned up {count} rate limit entries")
 
+        # Cleanup old audit logs
+        if now - _cleanup_times["audit"] >= _CLEANUP_INTERVALS["audit"]:
+            _cleanup_times["audit"] = now
+            if current_app.config.get("AUDIT_ENABLED", True):
+                from app.audit import cleanup_old_audit_logs
+
+                count = cleanup_old_audit_logs()
+                if count > 0:
+                    current_app.logger.info(f"Cleaned up {count} old audit log entries")
+
 
 from app.api import routes  # noqa: E402, F401