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

internal/db/migrations/001_schema.sql

@@ -3,14 +3,24 @@
 
 -- Source images, one row per source URL
 -- Files stored at: cache/sources/<ab>/<cd>/<source_key>, where source_key
--- is the SHA-256 of the source's host, path and query
+-- is the SHA-256 of the source's host, path and query.
+-- fetched_at is when the cached bytes were fetched. checked_at is when
+-- upstream last sent them or answered that they had not changed.
+-- upstream_etag and upstream_last_modified are the ETag and Last-Modified
+-- headers upstream sent with the bytes, or '' when it sent none.
 CREATE TABLE IF NOT EXISTS source_content (
     source_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,
-    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    fetched_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    checked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    upstream_etag TEXT NOT NULL DEFAULT '',
+    upstream_last_modified TEXT NOT NULL DEFAULT ''
 );
 CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
     ON source_content(last_accessed_at);
+CREATE INDEX IF NOT EXISTS idx_source_content_checked
+    ON source_content(checked_at);
 
 -- Processed variant blobs
 -- Files stored at: cache/variants/<ab>/<cd>/<cache_key>. Tracked here
@@ -18,14 +28,19 @@ CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
 -- without a directory scan, and so LRU eviction has a timestamp to order
 -- on. A cache hit does not read this table: it takes the content type
 -- from the file's first bytes, and its last_accessed_at is written
--- later, in a batch.
+-- later, in a batch. checked_at is the checked_at of the source the
+-- variant was made from, when it was made or last renewed; the variant
+-- file's modification time holds the same time for cache hits.
 CREATE TABLE IF NOT EXISTS variant_content (
     cache_key TEXT PRIMARY KEY,
     size_bytes INTEGER NOT NULL,
-    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    checked_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
 CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
     ON variant_content(last_accessed_at);
+CREATE INDEX IF NOT EXISTS idx_variant_content_checked
+    ON variant_content(checked_at);
 
 -- Total cache usage: the sum of size_bytes over source_content and
 -- variant_content, kept by the triggers below in the same statement