feat(dates): backfill drain converts mixed-convention rows to true instants (#118)

Per-tick drain (backfill_utc_dates) at the existing date-drain seam,
inside the per-library online gate. Selects date_utc IS NULL AND
date_taken IS NOT NULL (new partial index idx_image_exif_utc_backfill),
re-runs resolve_dates_batch with empty priors (one exiftool subprocess
per batch, same call shape as the #117 audit), and writes only the
new-contract triple; the legacy pair stays frozen. Capped by
DATE_BACKFILL_MAX_PER_TICK; missing-file rows are paged past without
spending the cap. Legacy manual overrides convert via
manual_date_resolution rather than being re-resolved from the file.

Day-boundary invalidation deletes the day-span precomputed reels
covering a re-dated row's old/new display-zone month-day so the nightly
pre-gen regenerates them. Progress: imageserver_date_backfill_remaining
gauge + date_backfill_remaining on GET /libraries.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

CLAUDE.md

@@ -437,6 +437,26 @@ a stronger tool comes online (exiftool install, new filename regex),
 re-resolve out-of-band rather than re-introducing the steady-state
 eligibility.
 
+**True-instant drain (`backfill_utc_dates`, ticket #118).** Runs right
+after `backfill_missing_date_taken` inside the per-library online gate,
+sharing its `DATE_BACKFILL_MAX_PER_TICK` cap. Converts legacy
+mixed-convention rows (`date_utc IS NULL AND date_taken IS NOT NULL`,
+partial index `idx_image_exif_utc_backfill`) by re-running
+`resolve_dates_batch` with empty priors (one exiftool subprocess per
+batch) and writing **only** the new-contract triple — the legacy pair
+stays frozen. Legacy `manual` rows convert via
+`camera_tz::manual_date_resolution` instead of being re-resolved from
+the file. Missing-file rows are paged past without spending the cap;
+EXIF/exiftool-legacy rows wait when exiftool is absent (they'd otherwise
+fall to fs_time). When a converted row's display-zone day differs from
+the day its legacy value rendered on, the `day`-span `precomputed_reels`
+rows covering either month-day (this library + `all`) are deleted and
+unlinked; the nightly pre-gen regenerates. Daily conversation summaries
+are SMS-derived and unaffected. Progress: Prometheus
+`imageserver_date_backfill_remaining` (per tick) and
+`date_backfill_remaining` per library on `GET /libraries` (live count of
+`date_utc IS NULL`).
+
 `/memories` is a single SQL query against this column
 (`get_memories_in_window` in `src/database/mod.rs`), using
 `strftime('%m-%d' | '%W' | '%m', date_taken, 'unixepoch', tz)` for