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

internal/imgcache/eviction.go

@@ -51,11 +51,6 @@ const sqliteTimestampLayout = "2006-01-02 15:04:05"
 // writes (".tmp-*" patterns in the storage layer).
 const tempFilePrefix = ".tmp-"
 
-// fallbackContentType is the content type reconciliation records for a
-// variant file it adopts whose first bytes are not those of an output
-// format. Such a file is never served.
-const fallbackContentType = "application/octet-stream"
-
 // UsageBytes returns the total number of bytes of cache content
 // tracked in the database (source content blobs plus processed
 // variants). It reads the total the database keeps up to date as rows
@@ -590,25 +585,16 @@ func (c *Cache) reconcileVariantFile(
 }
 
 // adoptVariantFile inserts an accounting row for a variant file that
-// has none, using the file's size and modification time, and the content
-// type its first bytes show, as a hit reads it.
+// has none, using the file's size and modification time.
 func (c *Cache) adoptVariantFile(
 	ctx context.Context, info fs.FileInfo, cacheKey imagerequest.VariantKey,
 ) error {
 	modTime := info.ModTime().UTC().Format(sqliteTimestampLayout)
-	contentType := fallbackContentType
 
-	content, _, detectedType, err := c.variants.LoadWithContentType(cacheKey)
-	if err == nil {
-		_ = content.Close()
-		contentType = detectedType
-	}
-
-	_, err = c.db.ExecContext(ctx, `
-		INSERT INTO variant_content
-			(cache_key, size_bytes, content_type, created_at, last_accessed_at)
-		VALUES (?, ?, ?, ?, ?)
-	`, string(cacheKey), info.Size(), contentType, modTime, modTime)
+	_, err := c.db.ExecContext(ctx, `
+		INSERT INTO variant_content (cache_key, size_bytes, last_accessed_at)
+		VALUES (?, ?, ?)
+	`, string(cacheKey), info.Size(), modTime)
 	if err != nil {
 		return fmt.Errorf("failed to adopt variant file into accounting: %w", err)
 	}