Check cached sources with upstream after the cache TTL (closes #69, closes #83)

A cached source and its variants were served until eviction, so a
changed image at the same URL was never picked up.

Once a source was last checked 24 hours ago, the next request for one
of its images asks upstream with If-None-Match and If-Modified-Since,
from the ETag and Last-Modified it stored. A 304 renews the source and
its variant without a download or a conversion. New bytes replace the
source, and each variant made before them is converted again. A failed
check serves the cached image and puts the URL in the negative cache.
Sources and variants not checked successfully for 30 days are evicted.

A variant file's modification time is its source's check time, so a
hit still makes no database call.

Existing tests change only in call signatures. The test that asserted
two-day-old entries are served without asking upstream is replaced by
the new ones.

Model: opus-5-5

TODO.md

@@ -30,6 +30,20 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-11-11 a cached source is checked with upstream once it is 24 hours old
+  (closes #69, #83). Before, it and its variants were served until eviction, so
+  a changed image at the same URL was never picked up.
+    - `source_content` keeps the source's `fetched_at`, `checked_at`, and the
+      `ETag` and `Last-Modified` upstream sent.
+    - The check sends them as `If-None-Match` and `If-Modified-Since`. A 304
+      renews the source; new bytes replace it.
+    - A variant file's modification time is its source's `checked_at`, so a hit
+      still makes no database call. A variant made before the source's
+      `fetched_at` is converted again.
+    - A failed check serves the cached image and puts the source URL in the
+      negative cache, so upstream is asked at most once per 5 minutes.
+    - The eviction goroutine evicts sources and variants whose `checked_at` is
+      30 days old (`CacheMaxAge`).
 - 2026-10-27 the reconciliation pass skips a directory it cannot list and a file
   whose info it cannot read, and goes on (closes #343). It stopped before its
   later checks at a directory it could not list, or at a source or variant file