Unusable DATA_DIR error names DATA_DIR; report file number is atomic.Int64 (closes #148, closes #161)

When the backend runs on its own, a DATA_DIR that could not be created
or read stopped the start with an error that named the path but not
DATA_DIR, and one where no file could be created let it start, to fail
at the first write of a report file. The start now creates a file in
DATA_DIR and removes it, and all three cases fail with
DATA_DIR "<value>": <reason>, the form config.New uses for the other
settings. Tests start the buffer with DATA_DIR below a regular file and
with DATA_DIR set to /proc.

The report file sequence counter is atomic.Int64 instead of
atomic.Uint64, as the Go style guide asks for int types. File names are
unchanged.

Model: opus-5-5

internal/reportbuf/reportbuf.go

@@ -134,9 +134,11 @@ func New(
 				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)
 			}
 
-			// Creating and removing a file now stops the start if
-			// report files cannot be written, instead of the first
-			// write failing after reports were answered 200.
+			// Creating and removing a file stops the start when no
+			// file can be created in DATA_DIR, instead of the first
+			// write of a report file failing after reports were
+			// answered 200. A full disk is not caught: the empty
+			// file is still created.
 			f, err := os.CreateTemp(b.dataDir, "")
 			if err != nil {
 				return fmt.Errorf("DATA_DIR %q: %w", b.dataDir, err)