feat(modelwatch): push producer for open-weight model releases

Add newsfeed-modelwatch: a one-shot, systemd-timer-driven producer that watches
the Hugging Face Hub for open-weight releases and pushes filtered candidates to
the ingest endpoint. Answers "how do I automate the HF firehose into my feed" —
the RSS half already works on the pull rail; this is the JSON/API half.

How it works: each run polls the HF models API (a watchlist of orgs +
trendingScore), applies an admission predicate (license allowlist, total-params
cap, safetensors present, not gated, pipeline_tag), and POSTs the survivors to
POST /v1/ingest/candidates under a bearer token, tagged source="huggingface".

Two properties of the existing ingest rail shape the design:
- Ingest is idempotent on (user, external_id). Using the repo id as external_id
  makes the producer STATELESS — no dedup table; re-runs/overlapping timers just
  re-submit and the server drops repeats.
- HF `tags` are copied onto the candidate, so the user's per-interest weights do
  the ranking. The predicate is only an ADMISSION filter (what's worth surfacing
  at all) and stays subordinate to explicit weights, per the house rule.

Layout: newsfeed-modelwatch is a client of the API, not an internal component —
it holds no core/data deps. Pure logic (HF types, predicate, mapping to
CandidateSubmission, param formatting) lives in the lib with unit tests; the bin
does the HTTP polling/posting. CandidateSubmission gains Serialize so the
in-workspace producer can build and post one.

Ops:
- asset/systemd/newsfeed-modelwatch.{service,timer}: oneshot + 3-hourly timer.
  The ingest token is a secret, kept in /etc/newsfeed/modelwatch.env
  (NEWSFEED_TOKEN, mapped to `token` by figment) so a redeploy of the config
  never clobbers it.
- asset/config/modelwatch.toml.tmpl: watchlist + predicate, no secret.
- infra-setup.sh installs the units, creates the env placeholder (never
  overwritten), enables the timer, and prints the token step.
- deploy.yml builds + ships the binary and the (secret-free) config each deploy.

Verified: unit tests (gated union, license precedence, admit accept/reject,
mapping); dry-run against live HF; full loop against a local API — minted token,
submitted a real release, confirmed it landed in the feed with the huggingface
source linked and HF tags carried through. fmt/clippy/test all green.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_016fKZzDpvjiJ9eYbPGgJvUP

asset/systemd/newsfeed-modelwatch.service

@@ -0,0 +1,32 @@
+[Unit]
+Description=newsfeed model-release watcher (Hugging Face Hub -> ingest)
+Documentation=https://git.lair.cafe/grenade/newsfeed
+After=network-online.target newsfeed-api.service
+Wants=network-online.target
+# Timer-activated (see newsfeed-modelwatch.timer); no [Install] here.
+
+[Service]
+Type=oneshot
+User=newsfeed
+Group=newsfeed
+# The ingest token (NEWSFEED_TOKEN) lives here, kept out of the deployable config so a
+# redeploy of modelwatch.toml never clobbers it.
+EnvironmentFile=/etc/newsfeed/modelwatch.env
+ExecStart=/usr/local/bin/newsfeed-modelwatch --config /etc/newsfeed/modelwatch.toml
+
+# Hardening (architecture generic.md §8). No DB access — this talks HTTP only, so no
+# ReadWritePaths are granted.
+NoNewPrivileges=true
+ProtectSystem=strict
+ProtectHome=true
+PrivateTmp=true
+PrivateDevices=true
+ProtectKernelTunables=true
+ProtectKernelModules=true
+ProtectControlGroups=true
+RestrictRealtime=true
+RestrictSUIDSGID=true
+LockPersonality=true
+MemoryDenyWriteExecute=true
+SystemCallArchitectures=native
+RestrictAddressFamilies=AF_UNIX AF_INET AF_INET6