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/__init__.py

@@ -190,6 +190,11 @@ def setup_metrics(app: Flask) -> None:
         metrics.info("flaskpaste_info", "FlaskPaste application info", version=VERSION)
 
         app.extensions["metrics"] = metrics
+
+        # Setup custom metrics
+        from app.metrics import setup_custom_metrics
+
+        setup_custom_metrics(app)
     except ImportError:
         app.logger.warning("prometheus_flask_exporter not available, metrics disabled")