feat(flashcard): add apkg-to-orgdrill.py, the inverse of flashcard-to-anki

The flashcard pipeline was one-directional (org-drill to apkg). A deck curated on the phone, or an orphaned apkg whose org source was never saved, had no way back into the org source of truth. apkg-to-orgdrill.py closes the loop.

An apkg is a zip holding an Anki sqlite collection, so reading it needs no third-party library: stdlib zipfile + sqlite3. genanki is only needed to write apkgs, not read them, so the converter has no runtime deps. It mirrors flashcard-to-anki.py inverted: deck name to #+TITLE, note Front to the ** heading, note Back HTML to the body (<br> to newlines, entities unescaped, the answer <hr> stripped), the note tag to a * section, and a fresh :ID: UUID per card so the output is org-drill-valid.

The entity unescape runs in the order that inverts escape_html, which escaped & first, so & is unescaped last here. Otherwise a literally-escaped &lt; in the source would wrongly collapse to <. Only Front/Back note types convert; a cloze or other model is skipped with a warning rather than silently dropped, and a note referencing media is flagged since the org side has no media path.

17 tests cover the HTML-to-org conversion, the apkg read (single and multiple decks, Default-deck exclusion, non-basic skip, .anki2 vs .anki21, media flag), the org emission, and errors. The round-trip test closes the loop through flashcard-to-anki.py's own parse(): a known org source becomes an apkg fixture, converts back, and the recovered (front, back, tag) tuples match the original. Verified end to end against a real genanki-produced apkg too.

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