add vision analysis service with prompt versioning, response schema, and structured JSON output

.env.example

@@ -0,0 +1,9 @@
+SESSION_SECRET=replace-with-a-long-random-secret
+# Optional: reachable from the Express server, never from the browser.
+# SEARXNG_URL=http://searxng:8080
+WEB_SEARCH_TIMEOUT_MS=8000
+UPLOAD_DIR=/app/data/uploads
+UPLOAD_MAX_SIZE_MB=10
+UPLOAD_MAX_FILES=5
+UPLOAD_MAX_PIXELS=30000000
+VISION_MODEL=gemma3:4b