fix: split token count columns into separate migration

A previous commit added prompt_eval_count and eval_count to the
existing 2026-05-27-000002_add_insight_generation_params migration,
but Diesel won't re-run an already-applied migration. Environments
that applied the original version of 000002 never got these two
columns, causing "no such column: photo_insights.prompt_eval_count"
on every insight read.

- Revert 000002 up.sql to its original 7-column form
- Add 000003_add_insight_token_counts for the two missing columns

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

migrations/2026-05-27-000003_add_insight_token_counts/down.sql

@@ -0,0 +1,13 @@
+-- SQLite doesn't support DROP COLUMN before 3.35.0; recreate the table
+-- without the token-count columns. This is only needed for rollback.
+CREATE TABLE photo_insights_old AS
+    SELECT id, library_id, rel_path, title, summary, generated_at,
+           model_version, is_current, training_messages, approved,
+           backend, fewshot_source_ids, content_hash,
+           num_ctx, temperature, top_p, top_k, min_p,
+           system_prompt, persona_id
+    FROM photo_insights;
+
+DROP TABLE photo_insights;
+
+ALTER TABLE photo_insights_old RENAME TO photo_insights;

migrations/2026-05-27-000003_add_insight_token_counts/up.sql

@@ -0,0 +1,6 @@
+-- Persist token usage on each insight row. Split from
+-- 2026-05-27-000002_add_insight_generation_params because that
+-- migration was already applied on some environments before these
+-- columns were added.
+ALTER TABLE photo_insights ADD COLUMN prompt_eval_count INTEGER;
+ALTER TABLE photo_insights ADD COLUMN eval_count INTEGER;