Add Google Gemini AI support (v1.0.6)

- Add Gemini as new AI provider option alongside Anthropic, OpenAI, Ollama
- Support models: Gemini 2.5 Flash Lite (default), Flash, Pro, 3 Flash Preview
- Add test API key endpoint to verify connection before saving
- Full support for extraction, verification, stock status, and arbitration
- Update all version files to 1.0.6

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "priceghost-backend",
-  "version": "1.0.5",
+  "version": "1.0.6",
   "description": "PriceGhost price tracking API",
   "main": "dist/index.js",
   "scripts": {
@@ -11,6 +11,7 @@
   },
   "dependencies": {
     "@anthropic-ai/sdk": "^0.24.0",
+    "@google/generative-ai": "^0.24.1",
     "axios": "^1.6.0",
     "bcrypt": "^5.1.1",
     "cheerio": "^1.0.0-rc.12",