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

Cached images were served until eviction, so upstream changes were
missed.

After 24 hours a request checks the source with upstream, sending its
stored ETag and Last-Modified. A 304 renews it with no download or
conversion. New bytes replace it; older variants are converted again.
A failed check serves the cached image and puts the URL in the
negative cache. Images not checked for 30 days are evicted.

Variant rows keep the source's ETag and Last-Modified for checking a
variant of an evicted source.

A variant file's modification time is its source's check time when it
was made or renewed. Hits read it without a database call. The ETag
includes it, so new bytes get a new ETag.

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 +28,24 @@ 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.
+-- upstream_etag and upstream_last_modified are those of the source bytes
+-- the variant was made from, so the variant can still be checked with
+-- upstream after its source is evicted.
 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,
+    upstream_etag TEXT NOT NULL DEFAULT '',
+    upstream_last_modified TEXT NOT NULL DEFAULT ''
 );
 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