feat: AI pre-fill for product form via Gemini API

Add POST /products/parse-text endpoint that accepts raw product text,
calls Gemini (google-genai) with a structured extraction prompt, and
returns a partial ProductParseResponse. Frontend gains a collapsible
"AI pre-fill" card at the top of ProductForm that merges the LLM
response into all form fields reactively.

- Backend: ProductParseRequest/Response schemas, system prompt with
  enum constraints, temperature=0.0 for deterministic extraction,
  effect_profile always returned in full
- Frontend: parseProductText() in api.ts; controlled $state bindings
  for all text/number/checkbox inputs; applyAiResult() merges response

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/pyproject.toml

@@ -6,6 +6,7 @@ readme = "README.md"
 requires-python = ">=3.12"
 dependencies = [
     "fastapi>=0.132.0",
+    "google-genai>=1.65.0",
     "psycopg>=3.3.3",
     "python-dotenv>=1.2.1",
     "sqlmodel>=0.0.37",