feat(ai): hybrid backend mode for agentic insights

Adds a `backend` column to photo_insights (default 'local', migration
2026-04-20-000000) and a corresponding optional `backend` field on the
agentic request. When a request sets backend=hybrid:

- The local Ollama vision model is called once via describe_image to
  produce a text description.
- The description is inlined into the first user message as text —
  no base64 image is ever sent to the chat model.
- The agentic tool-calling loop and title generation route through an
  OpenRouterClient (dispatched via &dyn LlmClient), letting the user
  pick any tool-capable model from OpenRouter per request.
- describe_photo is removed from the offered tools since the description
  is already present.

Embeddings and vision stay on local Ollama regardless of backend.
Hybrid mode requires OPENROUTER_API_KEY; handlers return a clear error
when hybrid is requested without it, and also when the selected
OpenRouter model lacks tool-calling support.

AppState gains an optional openrouter client built from
OPENROUTER_API_KEY / OPENROUTER_BASE_URL / OPENROUTER_DEFAULT_MODEL /
OPENROUTER_EMBEDDING_MODEL / attribution headers. Default model is
anthropic/claude-sonnet-4.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/bin/populate_knowledge.rs

@@ -134,6 +134,7 @@ async fn main() -> anyhow::Result<()> {
 
     let generator = InsightGenerator::new(
         ollama,
+        None,
         sms_client,
         insight_dao.clone(),
         exif_dao,
@@ -249,6 +250,7 @@ async fn main() -> anyhow::Result<()> {
                 args.top_k,
                 args.min_p,
                 args.max_iterations,
+                None,
             )
             .await
         {