daemon: move the Discord gateway and scheduler out of the web process into Go

web/app.py started two long-lived background jobs under a leader election: the
Discord gateway bot and an APScheduler. Both are stateful I/O loops -- reconnect,
RESUME, heartbeat, backoff, interval timers -- living inside an async web app
that also has to serve requests. This moves them into a single Go binary.

Go owns ONLY the stateful I/O. It owns no climate or grading logic: anything
needing data calls back into Python over a new internal-only HTTP surface
(/internal/discord/grade, /internal/jobs/warm-cities, /internal/jobs/indexnow).
Grading depends on polars and the parquet cache; reimplementing it in Go would
make the bot's grades drift from the API's, and the slash-command path
deliberately shares one grade builder so the two can never disagree. The grade
route returns gateway-ready JSON -- including the ephemeral-flag drop that
discord_bot.py used to do -- and Go relays those bytes verbatim without parsing
the embed.

Packaging: the binary is built by a golang:1.26 stage in the backend Dockerfile
and shipped in the SAME image, run as a second compose service off the SAME tag.
The daemon and backend share the /internal/* contract, so they must never skew
versions; one image makes that structural rather than a convention. Its
entrypoint bypasses entrypoint.sh -- the backend owns alembic, and two racing
migrators is a real hazard.

replicas: 1 in the Swarm stack is load-bearing. Discord permits exactly one
gateway connection per bot token; the pin replaces core/singleton.claim_leader
for this workload. update_config uses order: stop-first, since start-first would
briefly run two gateways. autoscale.sh targets ${STACK_NAME}_web only, so it
cannot scale this.

Security: the internal routes compare the token with hmac.compare_digest and the
whole router 404s when THERMOGRAPH_INTERNAL_TOKEN is unset -- fail closed, never
default open. Caddy only routes /api/*, /digest and /discord/interactions to the
backend, so /internal/* was never publicly reachable; the token is defence in
depth. The router mounts before the catch-all frontend proxy so /internal/*
cannot fall through to it. The daemon refuses to start without the token.

Behaviour preserved from the Python, with the reasoning carried into the Go
comments: non-privileged intents (no MESSAGE_CONTENT, so no portal review);
fatal close codes 4004/4010-4014 stop rather than loop; the bot-author and
self-author mention-loop guard; allowed_mentions locked to {"parse":[],
"replied_user":true} so a crafted query cannot turn a reply into an @everyone
ping; the first cron tick deferred one full interval rather than firing at boot,
since warm-cities already runs at deploy time; and no overlapping warm-cities
run, which would double-spend the archive-fetch quota.

Two deliberate improvements over the Python. A close intended for RESUME now
uses 4000 rather than 1000 -- Discord invalidates a session closed 1000/1001, so
the Python's default close silently defeated its own resume. And MESSAGE_CREATE
is handled on a bounded worker pool rather than an unbounded thread hand-off, so
a flood of mentions cannot spawn unbounded work against the backend.

A .dockerignore is added because a disposable backend/.venv was being swallowed
by COPY . /app/ and duplicated again by the chown layer, inflating the image to
1.8 GB; it builds at 578 MB.

Tests: 29 Go gateway tests covering every behaviour the deleted
test_discord_bot.py asserted, plus cron/config/apiclient suites; 10 new Python
tests for the internal routes (fail-closed, auth, flag drop, per-job 409 guard).
Full suite 359 passed / 7 skipped; go build, vet and test -race clean.

infra/deploy/deploy.sh

@@ -235,6 +235,30 @@ if [ "$pull_ok" != 1 ]; then
   exit 1
 fi
 
+# The daemon binary ships INSIDE the backend image, but infra and app images
+# advance on different axes by design: this checkout tracks `main`, while image
+# tags are env-staged (prod's come from `release`). So a host can legitimately
+# be asked to roll a backend image OLDER than this compose file -- one built
+# before the daemon existed and with no /usr/local/bin/thermograph-daemon in it.
+# Creating the service anyway would leave a container crash-looping on a missing
+# binary, on a deploy that otherwise succeeded. Probe the image we are actually
+# about to roll and drop the daemon from this run if it can't support it; the
+# next deploy of a tag that has it picks it up with no further action.
+case " ${TARGETS[*]} " in
+  *" daemon "*)
+    daemon_img="$(docker compose config --images daemon 2>/dev/null | head -1 || true)"
+    if [ -n "$daemon_img" ] \
+       && ! docker run --rm --entrypoint sh "$daemon_img" -c 'test -x /usr/local/bin/thermograph-daemon' 2>/dev/null; then
+      echo "==> $daemon_img predates the daemon binary; rolling without the daemon service this run"
+      kept=()
+      for t in "${TARGETS[@]}"; do
+        [ "$t" = daemon ] || kept+=("$t")
+      done
+      TARGETS=("${kept[@]}")
+    fi
+    ;;
+esac
+
 # Roll only the target service(s). Backend schema migrations run inside its own
 # entrypoint (alembic upgrade head) before uvicorn, so there's no separate
 # migrate step; frontend is stateless.