api: serve PDFs — extraction in the child under the pool, full text or map, errors

Route "pdf" through clean.Kind in doFetch/cleanFetched: extract under the pool
(keyed by caller class + document hash, one per caller), map the five pdftext
errors plus a pool panic to 422, ErrBusy to 503+Retry-After 10, ErrCallerBusy to
429, and answer with the whole text or the map. Adds RetryAfter to fetch.Error
and the matching header. No cache, pages or grep — those are task 07.

Co-Authored-By: ornith-1.5-35b-a3b <ornith-1.5-35b-a3b@llama.cpp.invalid>

internal/fetch/fetch.go

@@ -43,9 +43,10 @@ func (r *Resource) SHA256() string {
 
 // Error is a fetch failure with the HTTP status sift should return to its caller.
 type Error struct {
-	Status int
-	Msg    string
-	Hint   string
+	Status     int
+	Msg        string
+	Hint       string
+	RetryAfter int // seconds to wait before retrying; 0 = none
 }
 
 func (e *Error) Error() string { return e.Msg }