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/config.py

@@ -100,6 +100,11 @@ class Config:
     # Authenticated users get higher limits (multiplier)
     RATE_LIMIT_AUTH_MULTIPLIER = int(os.environ.get("FLASKPASTE_RATE_AUTH_MULT", "5"))
 
+    # Audit Logging
+    # Track security-relevant events (paste creation, deletion, rate limits, etc.)
+    AUDIT_ENABLED = os.environ.get("FLASKPASTE_AUDIT", "1").lower() in ("1", "true", "yes")
+    AUDIT_RETENTION_DAYS = int(os.environ.get("FLASKPASTE_AUDIT_RETENTION", "90"))
+
     # PKI Configuration
     # Enable PKI endpoints for certificate authority and issuance
     PKI_ENABLED = os.environ.get("FLASKPASTE_PKI_ENABLED", "0").lower() in ("1", "true", "yes")