Split edition parts filled before #2915 stay Wanted and keep being searched #3048

Problem

#2915 stops series fill from offering the parts of a split edition (Stormlight's "Part 1" and "Part 2" releases) when you already own the whole book. That only helps installs that never filled the series before it lands.

Anyone who pressed Fill on such a series earlier already has the part rows as local books. They bind as Present by foreign ID, so they are never treated as covered, and:

• Fill re-queues every series book that is not imported (the ListBooksInSeries loop in Fill, internal/api/series.go), so the parts go back to Wanted on every fill.
• The wanted sweep keeps searching them and can download a part of a book you already own.

Proposed fix

Apply the same two signal rule #2915 uses (fractional position under an owned whole, title is the whole's title plus "Part N") to existing local rows: skip them in Fill's requeue and in the wanted sweep, and offer a one click "unmonitor split parts of owned books" on the series page so users can clean up. Never delete rows automatically.

Refs #2524, #2915.