feat(ai): few-shot exemplars + sticky Ollama preference

- Few-shot injection on /insights/generate/agentic: compresses prior
  training_messages into trajectory blocks (tool calls + result summaries)
  and injects into the system prompt. Hardcoded default ids with optional
  request override.
- New fewshot_source_ids column on photo_insights (+ migration) to track
  which exemplars influenced a given row, for downstream training-set
  filtering. Chat amend rows stamp None with a lineage note.
- Ollama client now remembers which server (primary/fallback) most
  recently succeeded and tries it first on the next call, via a shared
  Arc<AtomicBool>. Avoids re-404ing the primary on every agent iteration
  when the chosen model only lives on the fallback.
- Demote noisy logs: daily_summary "Summary match" lines to debug;
  inner chat_with_tools non-2xx body log from error to warn (outer
  layer owns the terminal-error signal).
- Drift-guard tests for summarize_tool_result covering the success /
  empty / error / unknown shape for every tool.
- Tidy: three pre-existing clippy warnings cleaned up.

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

migrations/2026-04-24-000000_add_fewshot_source_to_insights/down.sql

@@ -0,0 +1,24 @@
+-- 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, backend
+    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,
+    backend TEXT NOT NULL DEFAULT 'local'
+);
+INSERT INTO photo_insights
+    SELECT id, library_id, rel_path, title, summary, generated_at, model_version,
+           is_current, training_messages, approved, backend
+    FROM photo_insights_backup;
+DROP TABLE photo_insights_backup;

migrations/2026-04-24-000000_add_fewshot_source_to_insights/up.sql

@@ -0,0 +1 @@
+ALTER TABLE photo_insights ADD COLUMN fewshot_source_ids TEXT;