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/config/modelwatch.toml.tmpl

@@ -0,0 +1,35 @@
+# newsfeed-modelwatch configuration (production). Rendered to /etc/newsfeed/modelwatch.toml
+# by the deploy workflow.
+#
+# The ingest token is NOT here — it is a secret, supplied via NEWSFEED_TOKEN in
+# /etc/newsfeed/modelwatch.env (created once by infra-setup.sh). That keeps redeploys of
+# this file from ever clobbering the token.
+
+ingest_url = "http://127.0.0.1:22672/v1/ingest/candidates"
+source_name = "huggingface"
+
+# HF orgs to watch for new open-weight repos.
+authors = [
+    "tencent", "Qwen", "deepseek-ai", "mistralai", "zai-org", "moonshotai",
+    "openbmb", "allenai", "nvidia", "ibm-granite", "google", "meta-llama", "microsoft",
+]
+
+# The trending list is the best built-in noise filter; a notable release surfaces fast.
+include_trending = true
+trending_limit = 25
+per_author_limit = 15
+# Ignore repos older than this so the first run doesn't backfill old "newest" repos.
+max_age_days = 30
+
+http_timeout_secs = 20
+user_agent = "newsfeed-modelwatch (+https://git.lair.cafe/grenade/newsfeed)"
+
+# Admission filter: what's worth putting in front of you at all. Ranking is NOT decided
+# here — the HF tags ride onto each candidate so your per-interest weights do the ranking.
+[predicate]
+licenses = ["apache-2.0", "mit"]
+# Total safetensors params. NB: this is total, not MoE-active (that needs config.json math).
+max_params = 40000000000
+require_safetensors = true
+skip_gated = true
+pipeline_tags = ["text-generation"]