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/database/models.rs

@@ -100,6 +100,8 @@ pub struct InsertPhotoInsight {
     pub model_version: String,
     pub is_current: bool,
     pub training_messages: Option<String>,
+    /// `"local"` (Ollama with images) | `"hybrid"` (local vision + OpenRouter chat).
+    pub backend: String,
 }
 
 #[derive(Serialize, Queryable, Clone, Debug)]
@@ -115,6 +117,8 @@ pub struct PhotoInsight {
     pub is_current: bool,
     pub training_messages: Option<String>,
     pub approved: Option<bool>,
+    /// `"local"` (Ollama with images) | `"hybrid"` (local vision + OpenRouter chat).
+    pub backend: String,
 }
 
 // --- Libraries ---