Instapaper connector

[brendanlefebvre]

Add an Instapaper connector so Instapaper users can serve their queue as an OPDS catalog.

Why

Second half of the README's "more connectors (Instapaper, Wallabag)" promise. Shipping Wallabag + Instapaper together makes multi-backend support the 1.0 headline rather than a promissory note.

Scope

• Implement connectors/instapaper.py against connectors/base.py.
• Instapaper's Full API uses OAuth 1.0a (xAuth) — consumer key/secret + username/password → access token. Handle signing.
• Folders: Instapaper has Unread / Archive / Starred + user-defined folders. Map the built-ins at minimum.
• Article HTML: Instapaper's bookmarks/get_text returns cleaned article HTML — use it for get_article_html.
• Note: Full API access requires Instapaper subscription/approval; document this caveat.
• Config: INSTAPAPER_CONSUMER_KEY/SECRET, INSTAPAPER_USERNAME/PASSWORD in .env.example + config.py.

Acceptance criteria

• Instapaper queue browsable via OPDS; articles download as EPUB.
• Pagination via the connector cursor contract.
• Tests mirroring test_readwise.py (mocked HTTP).
• README + .env.example updated, including the API-access caveat.