package: add Pushover to the send2 toolkit

[WLTB-Gino]

Adds send2pushover, a Pushover (pushover.net) notifier following the send2ntfy/send2gotify pattern.

[themactep]

Thanks for this — the implementation follows the send2ntfy/send2gotify pattern cleanly and every integration point is wired correctly (script, send2.json default, json-send2.cgi GET/POST/default, send.cgi whitelist, the three motion bridges, agent-adapter, and the WebUI page/JS/table/capabilities). Marking video: false and using --form-string for all text fields are both right. A few things worth addressing before merge:

1. Branch is stale — rebase needed. This is 12 commits behind master, and f6311f43f ("motion: suppress events while pan/tilt motors are active") touched three of the same files: package/prudynt-t/files/motion, package/thingino-raptor/files/raptor-motion, and package/timps/files/timps-motion. The hunks sit in non-overlapping regions so they'd likely apply cleanly, but please rebase and re-test, especially the prudynt motion script (master added a MOTOR_ACTIVE_FLAG guard this branch doesn't have).

2. Verbose mode exposes the full credential pair. echo_command redacts the preview (token=***/user=***), but the actual curl --verbose invocation dumps the multipart request body to stderr — token and user key in cleartext. send.cgi's "wait for output" test path captures stderr and renders it in the browser. This is pre-existing across the toolkit (gotify leaks the token in the URL, ntfy in the Authorization header), but Pushover sends both credentials in the body, so both get dumped. Worth a note or a follow-up.

3. Priority edge cases.

• Help text advertises -2 lowest, 2 highest, but priority 2 (Emergency) requires retry/expire parameters, which the script never sends — send2pushover -z 2 gets a 400. The WebUI correctly caps at 1; the CLI help should match.
• No range validation on priority at all (same laxness as gotify, but a bad value just fails at the API).

4. 2.5 MB attachment limit is unguarded. Pushover rejects image attachments over 2.5 MB; the script doesn't check or resize. Higher-res sensors (4K+) can produce JPEGs over that, in which case photo notifications fail via the generic error path. The WebUI note documents the limit, but there's no guard. Has this been tested against a real camera snapshot?

Nits (non-blocking):

• shfmt -i 0 -ci reformats the double-tab continuation indent — send2gotify/send2ntfy share the same style, so it's consistent, just noting it.
• tool-send2-pushover.js reads send_video into a disabled checkbox on load but always writes false on save.
• send2.json block lands between mqtt and ntfy (file's only loosely sorted, so no big deal).

Only item 1 blocks merge; 2–4 are worth fixing or acknowledging.