Eviction no longer reads a whole table while requests wait on the database (closes #227)

UsageBytes now reads the new cache_usage row, which triggers on
source_content and variant_content keep up to date in the statement
that adds, removes or resizes a row. The reconciliation pass reads both
tables 1000 rows per query, sums them, and corrects the total when it
differs, unless a row changed while it summed. Source rows now get
last_accessed_at when added, so choosing source images to evict reads
that column's index instead of sorting the whole table. Stats still
sums the tables, now in pages: an existing test drops both tables and
expects that sum to fail.

Model: opus-5-5

internal/imgcache/cachesize.go

@@ -72,7 +72,7 @@ func (c *Cache) computeDefaultMaxBytes(
 	}
 
 	// Both terms are at most math.MaxInt64, so the sum cannot overflow.
-	//nolint:gosec // G115: UsageBytes sums file sizes, never negative
+	//nolint:gosec // G115: UsageBytes returns the total cache usage, never negative
 	spaceBytes := min(freeBytes, math.MaxInt64) + uint64(usedBytes)
 
 	computed := spaceBytes / freeSpaceFractionDenominator * freeSpaceFractionNumerator