Merge branch 'main' into golangci-v2.12.2

Absorbs the cache size management and LRU eviction work (#55). All four
textual conflicts resolved in favor of main's implementation, with this
branch's mechanical lint conformance re-applied on top:

- internal/config/config.go: took main's cache_max_bytes wiring
  (CacheMaxBytes, cacheMaxBytesExplicit) verbatim and expressed the key
  through this branch's constant convention as keyCacheMaxBytes.
- internal/imgcache/cache.go: took main's disabled-cache guards, LRU
  touch on lookup, and variant_content size accounting verbatim;
  re-applied this branch's signature wrapping for lll.
- internal/imgcache/storage.go: took main's new writeIfAbsent helper and
  its temp-file cleanup defer verbatim. The auto-merge had silently
  dropped that defer in favor of this branch's inline cleanup form;
  restored so the cleanup semantics that arrived from main are intact.
- TODO.md: kept both sides' Completed Steps entries.

.golangci.yml resolves to this branch's canonical version
(sha256 021cc83f4e6fc7c31b95b34b846723dfcf20b66b7baeea1dc40406e643346bcb).

make build and make test are green; #55's code is not yet conformant
with the canonical lint config, which the following commits address.

internal/database/schema/001_initial_schema.sql

@@ -3,12 +3,17 @@
 
 -- Source content blobs
 -- Files stored at: cache/src-content/<ab>/<cd>/<sha256>
+-- last_accessed_at is NULL until the first LRU touch; eviction falls
+-- back to fetched_at for rows that have never been touched.
 CREATE TABLE IF NOT EXISTS source_content (
     content_hash TEXT PRIMARY KEY,
     content_type TEXT NOT NULL,
     size_bytes INTEGER NOT NULL,
-    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP
+    fetched_at DATETIME DEFAULT CURRENT_TIMESTAMP,
+    last_accessed_at DATETIME
 );
+CREATE INDEX IF NOT EXISTS idx_source_content_last_accessed
+    ON source_content(last_accessed_at);
 
 -- Source URL metadata - maps URLs to content hashes
 -- JSON stored at: cache/src-metadata/<hostname>/<path_hash>.json
@@ -34,6 +39,22 @@ CREATE INDEX IF NOT EXISTS idx_source_meta_path_hash ON source_metadata(path_has
 CREATE INDEX IF NOT EXISTS idx_source_meta_expires ON source_metadata(expires_at);
 CREATE INDEX IF NOT EXISTS idx_source_meta_content_hash ON source_metadata(content_hash);
 
+-- Processed variant blobs
+-- Files stored at: cache/variants/<ab>/<cd>/<cache_key> (plus a .meta
+-- sidecar with the content type). Tracked here (like source content
+-- blobs above) so total cache usage can be computed with a SUM query,
+-- never a directory scan, and so LRU eviction has a timestamp to order
+-- on.
+CREATE TABLE IF NOT EXISTS variant_content (
+    cache_key TEXT PRIMARY KEY,
+    size_bytes INTEGER NOT NULL,
+    content_type TEXT NOT NULL,
+    created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    last_accessed_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
+);
+CREATE INDEX IF NOT EXISTS idx_variant_content_last_accessed
+    ON variant_content(last_accessed_at);
+
 -- Output/transformed content blobs
 -- Files stored at: cache/dst-content/<ab>/<cd>/<sha256>
 CREATE TABLE IF NOT EXISTS output_content (