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>

migrations/2026-04-20-000000_add_backend_to_insights/down.sql

@@ -0,0 +1,23 @@
+-- SQLite can't DROP COLUMN cleanly on older versions; rebuild the table.
+CREATE TABLE photo_insights_backup AS
+    SELECT id, library_id, rel_path, title, summary, generated_at, model_version,
+           is_current, training_messages, approved
+    FROM photo_insights;
+DROP TABLE photo_insights;
+CREATE TABLE photo_insights (
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
+    library_id INTEGER NOT NULL REFERENCES libraries(id),
+    rel_path TEXT NOT NULL,
+    title TEXT NOT NULL,
+    summary TEXT NOT NULL,
+    generated_at BIGINT NOT NULL,
+    model_version TEXT NOT NULL,
+    is_current BOOLEAN NOT NULL DEFAULT TRUE,
+    training_messages TEXT,
+    approved BOOLEAN
+);
+INSERT INTO photo_insights
+    SELECT id, library_id, rel_path, title, summary, generated_at, model_version,
+           is_current, training_messages, approved
+    FROM photo_insights_backup;
+DROP TABLE photo_insights_backup;

migrations/2026-04-20-000000_add_backend_to_insights/up.sql

@@ -0,0 +1 @@
+ALTER TABLE photo_insights ADD COLUMN backend TEXT NOT NULL DEFAULT 'local';