Eviction no longer reads a whole table while requests wait on the database (closes #227)

UsageBytes now reads the new cache_usage row, which triggers on
source_content and variant_content keep up to date in the statement
that adds, removes or resizes a row. The reconciliation pass reads both
tables 1000 rows per query, sums them, and corrects the total when it
differs, unless a row changed while it summed. Source rows now get
last_accessed_at when added, so choosing source images to evict reads
that column's index instead of sorting the whole table. Stats still
sums the tables, now in pages: an existing test drops both tables and
expects that sum to fail.

Model: opus-5-5

TODO.md

@@ -38,6 +38,14 @@ P2: security: per-IP rate limiting on the image routes
   save JPEG XL, so pixad does not start without it. A test saves an image as
   JPEG XL with govips and loads it back. JPEG XL is not yet a format pixa
   serves.
+- 2026-10-08 requests no longer wait behind eviction queries that read a whole
+  table (closes #227): the new `cache_usage` table holds the total cache usage,
+  kept up to date by triggers on `source_content` and `variant_content` in the
+  statement that adds, removes or resizes a row, and `UsageBytes` reads it. The
+  reconciliation pass reads the content tables 1000 rows per query, sums them
+  and corrects the total when it differs, unless a row changed while it summed.
+  Source rows get `last_accessed_at` when added, so choosing source images to
+  evict reads that column's index instead of sorting the whole table.
 - 2026-10-08 SQLite writes no longer fail with "database is locked" under load
   (closes #223): `internal/database` opens the database with one connection, so
   pixa's own reads and writes run on it one at a time instead of competing for