feat: add FlareSolverr proxy support with DB-backed config

- Add config table for storing FlareSolverr proxy setting
- Add HTTP endpoints to get/set proxy status (GET/POST /api/config/flaresolverr)
- Refactor httpclient to support proxy mode (requests go through FlareSolverr)
- Add verbose logging for debugging
- Add POST support to FlareSolverr client

Usage:
  GET /api/config/flaresolverr - returns {flaresolverr_proxy: bool}
  POST /api/config/flaresolverr - body: {enabled: true/false}

internal/db/queries/config.sql

@@ -0,0 +1,6 @@
+-- name: GetConfigValue :one
+SELECT value FROM config WHERE key = $1;
+
+-- name: SetConfigValue :exec
+INSERT INTO config (key, value) VALUES ($1, $2)
+ON CONFLICT (key) DO UPDATE SET value = EXCLUDED.value;