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-lock.json

@@ -1,14 +1,15 @@
 {
   "name": "priceghost-backend",
-  "version": "1.0.2",
+  "version": "1.0.5",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "priceghost-backend",
-      "version": "1.0.2",
+      "version": "1.0.5",
       "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",
@@ -531,6 +532,15 @@
         "node": ">=18"
       }
     },
+    "node_modules/@google/generative-ai": {
+      "version": "0.24.1",
+      "resolved": "https://registry.npmjs.org/@google/generative-ai/-/generative-ai-0.24.1.tgz",
+      "integrity": "sha512-MqO+MLfM6kjxcKoy0p1wRzG3b4ZZXtPI+z2IE26UogS2Cm/XHO+7gGRBh6gcJsOiIVoH93UwKvW4HdgiOZCy9Q==",
+      "license": "Apache-2.0",
+      "engines": {
+        "node": ">=18.0.0"
+      }
+    },
     "node_modules/@mapbox/node-pre-gyp": {
       "version": "1.0.11",
       "resolved": "https://registry.npmjs.org/@mapbox/node-pre-gyp/-/node-pre-gyp-1.0.11.tgz",