feat: AI photo analysis for skin snapshots

Add POST /skincare/analyze-photos endpoint that accepts 1–3 skin
photos, sends them to Gemini vision, and returns a structured
SkinPhotoAnalysisResponse for pre-filling the snapshot form.

Extract shared Gemini client setup into innercontext/llm.py
(get_gemini_client) so both products and skincare use a single
default model (gemini-flash-latest) and API key check.

Frontend: AI photo card on /skin page with file picker, previews,
and auto-fill of all form fields from the analysis result.
New fields (skin_type, sebum_tzone, sebum_cheeks) added to form
and server action.

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

backend/pyproject.toml

@@ -9,6 +9,7 @@ dependencies = [
     "google-genai>=1.65.0",
     "psycopg>=3.3.3",
     "python-dotenv>=1.2.1",
+    "python-multipart>=0.0.22",
     "sqlmodel>=0.0.37",
     "uvicorn[standard]>=0.34.0",
 ]