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/imagerequest.go

@@ -1,7 +1,7 @@
 // Package imagerequest holds the image request type, its parts, its checks,
-// the parsing of the /v1/image/ path and the variant key. It imports no other
-// package of this repo, so every package that handles an image request can
-// import it.
+// the parsing of the /v1/image/ path, the variant key and the source key. It
+// imports no other package of this repo, so every package that handles an
+// image request can import it.
 package imagerequest
 
 import (