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

@@ -30,6 +30,14 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 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 libvips' JPEG XL support is installed and required (part of #222):
   `script/bootstrap --cgo` installs `vips-jxl` when its package manager is apk,
   as Alpine's `vips` package lacks the support, and the runtime stage of the