Store a source the way a variant is stored, one row and one file per source URL (closes #308)

A source is now one source_content row and one file named by its
SourceKey, the SHA-256 of its host, path and query, made as the variant
key is. A store writes the file, then upserts the row, and removes the
file when the row cannot be written; an eviction deletes the row, then
the file, with no lock. source_metadata, the JSON files under
cache/metadata/, MetadataStorage, the content lock, the eviction
transaction and its test hook are gone, as is the reconciliation pass
that dropped a source row whose file is missing. Two source URLs
serving the same bytes store them twice.

Model: opus-5-5

internal/imagerequest/sourcekey.go

@@ -0,0 +1,26 @@
+package imagerequest
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"fmt"
+)
+
+// SourceKey is a SHA256 hash identifying a cached source image (hex-encoded).
+type SourceKey string
+
+// Key returns the key the source is cached under, made as CacheKey makes a
+// variant key.
+// Format: sha256(host:path:query)
+// The path and query go through escapePercentAndColon, so that a ":" in
+// either cannot be taken for the one between them.
+func (s Source) Key() SourceKey {
+	data := fmt.Sprintf("%s:%s:%s",
+		s.Host,
+		escapePercentAndColon(s.Path),
+		escapePercentAndColon(s.Query),
+	)
+	h := sha256.Sum256([]byte(data))
+
+	return SourceKey(hex.EncodeToString(h[:]))
+}