v1.39.1 — Retry symmetry: Discord + Webhook (audit)

Fixed

• Discord and generic webhook notifications could silently drop on a transient network blip — proactive audit follow-up to v1.38.1. _discord_post and _webhook_send did a single urlopen in a try/except, the same structural gap that ate NotRetarded's Telegram notification right after a self-update restart. Zero reports of Discord/webhook loss so far, but the code was one bad blip away from the same symptom on either channel. A shared _post_json_with_retry now covers all three channels: 3 attempts with 2s/4s backoff on transient network failures, no retry on HTTP status codes (server rejection ≠ transient blip).

Note: for a generic webhook pointing at an automation (Home Assistant, ntfy, custom script), the tiny edge case where a delivered send timed out on read can yield a duplicate delivery — prefer idempotent handlers.

Full 23-script regression sweep green.