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

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

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews the source with no download
or conversion. New bytes replace the source, and older variants are
converted again. A failed check serves the cached image and puts the
URL in the negative cache. This includes the full fetch made for a
variant whose source was evicted. Images not checked 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.

Model: opus-5-5

TODO.md

@@ -30,6 +30,22 @@ 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). 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. A variant
+      whose source was evicted is checked with a full fetch. When that fetch
+      fails, the cached variant is served the same way.
+    - The eviction goroutine evicts sources and variants whose `checked_at` is
+      30 days old (`EvictionAfterLastCheck`).
 - 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