Declare only the tables and columns pixa writes and reads (closes #310)

Removes output_content and request_cache, which nothing writes, and every
column that was written but never read: the host, path, query, content_type
and fetched_at of source_content, the content_type and created_at of
variant_content, and the status_code, error_message and fetched_at of
negative_cache.

StoreSource no longer takes the fetch result and LookupSource returns only
the source key. StoreVariant no longer takes a content type and
StoreNegative no longer takes a status code or message, so
extractStatusCode and the status constants, which only fed status_code,
are gone. Reconciliation adopts a variant file without opening it, as its
row no longer records a content type. Tests that set or checked a removed
column follow.

Model: opus-5-5

TODO.md

@@ -30,6 +30,15 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-21 the schema file declares only the tables and columns pixa writes
+  and reads (closes #310): `output_content` and `request_cache` are gone, and so
+  are the columns nothing read: the host, path, query, `content_type` and
+  `fetched_at` of `source_content`, the `content_type` and `created_at` of
+  `variant_content`, and the `status_code`, `error_message` and `fetched_at` of
+  `negative_cache`. `StoreSource` no longer takes the fetch result,
+  `LookupSource` returns only the source key, `StoreVariant` no longer takes a
+  content type, `StoreNegative` no longer takes a status code or message, and
+  reconciliation adopts a variant file without reading its first bytes.
 - 2026-10-21 a cache hit makes no database call (closes #309): `Service.Get`
   opens the variant file with `Cache.GetVariant` and nothing else, with no
   existence check first. The content type comes from the file's first bytes; a