memories: single-SQL rewrite + 20-year lookback

Replaces the EXIF-loop + WalkDir-fallback pipeline that powered
`/memories` with a single per-library SQL query
(`get_memories_in_window`) that uses `strftime('%m-%d' | '%W' | '%m',
date_taken, 'unixepoch', tz_offset)` for calendar matching in the
client's timezone, plus a `years_back` lower bound and a
no-future-dates upper bound. Returns only the matching rows; the
handler applies per-library `PathExcluder` post-query and sorts.

Drops:
- `collect_exif_memories` — replaced by the single SQL query.
- `collect_filesystem_memories` — the canonical-date pipeline now
  populates `date_taken` for every row at ingest, so the WalkDir
  fallback that scanned 14k+ files each request is no longer needed.
- `get_memory_date_with_priority` and friends — request-time waterfall
  superseded by `date_resolver` running at ingest. The associated
  three priority-tests are dropped; their replacement lives in
  `date_resolver::tests`.

On a ~14k-file library this drops `/memories` from 10–15 s
(dominated by `fs::metadata` per row) to single-digit ms.

Bumps `DEFAULT_YEARS_BACK` from 15 → 20 to surface deeper archives
on matching anniversaries.

Note vs. ISO weeks: the original Rust used `chrono::iso_week().week()`
for week-span matching. SQLite's `%W` is Monday-anchored but uses week
0 for days before the first Monday, so it can disagree with ISO at
year boundaries by ±1. Acceptable for nostalgia browsing.

Adds 3 new DAO tests covering month-span filter, library scoping, and
the unknown-span-token guard. Also adds a CLAUDE.md section describing
the canonical-date pipeline end-to-end and the new
`DATE_BACKFILL_MAX_PER_TICK` env var.

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

src/files.rs

@@ -1666,6 +1666,17 @@ mod tests {
             Ok(())
         }
 
+        fn get_memories_in_window(
+            &mut self,
+            _context: &opentelemetry::Context,
+            _library_id: i32,
+            _span_token: &str,
+            _years_back: i32,
+            _tz_offset_minutes: i32,
+        ) -> Result<Vec<(String, i64, i64)>, DbError> {
+            Ok(Vec::new())
+        }
+
         fn find_by_content_hash(
             &mut self,
             _context: &opentelemetry::Context,