Post the daily "most unusual right now" feed to Discord (#205)

Adds an optional daily Discord post of the day's most anomalous cities, built from
the same data/homepage.json feed the homepage renders — so the post and the site
always agree. Delivery is a single incoming-webhook POST (no bot, no gateway, no
new process): it rides the notifier daemon like the feed refresh does, once per
day after the refresh, leader-only. With no webhook configured it no-ops.

- backend/discord.py: build_embed() renders the top cities as a rich embed (each
  line: city, reading in °F and °C, the normal for context, percentile + grade;
  non-today readings are dated). post_daily_feed() is best-effort — it skips a
  stale/empty feed and never raises, so a Discord failure can't disturb a pass.
- notify.py: a once-a-day guard (_maybe_post_discord) beside the homepage-refresh
  guard; only marks the day done once a post lands, so a transient failure retries.
- deploy/thermograph.env.example: THERMOGRAPH_DISCORD_WEBHOOK (the URL is the
  credential — env only).

The webhook value is the top anomaly's tail for the accent colour and reuses the
feed's build time as the embed timestamp. httpx (already a dependency) does the POST.

Claude-Session: https://claude.ai/code/session_013dRZmX9D3JEntfMKWMTWZ8

deploy/thermograph.env.example

@@ -73,3 +73,12 @@ THERMOGRAPH_BASE_URL=https://thermograph.org
 # value, which would invalidate every outstanding link on each restart.
 #   generate with:  python -c "import secrets; print(secrets.token_urlsafe(48))"
 #THERMOGRAPH_AUTH_SECRET=
+
+# --- Discord ---------------------------------------------------------------------
+# Incoming webhook URL for the daily "most unusual right now" post. Create it in the
+# Discord server: Channel → Edit → Integrations → Webhooks → New Webhook → Copy URL.
+# The URL IS the credential — anyone who has it can post as the webhook, so keep it
+# here and never in the repo. Unset => the daily post is disabled (no-op).
+# The post rides the notifier daemon (leader-only), once per day after the feed
+# refresh, so it needs THERMOGRAPH_ENABLE_NOTIFIER on (the default).
+#THERMOGRAPH_DISCORD_WEBHOOK=