fix(modelwatch,fetch): param-count fallback + browser-shaped feed headers

Two fixes surfaced by the live feed.

modelwatch: some repos report a bogus tiny `safetensors.total` (e.g.
deepreinforce-ai/Ornith-1.0-35B came through as "1M params"), which both
misrendered and — worse — let an oversized model slip past the size cap. Add
`params_from_name` (parse the first <num>B|M token from the repo id, which by
convention is the total size) and `effective_params` (trust safetensors.total
only when it's >= 100M, else fall back to the name). Use it for the size filter
and the summary. New unit tests cover the name parser, the fallback, and that a
70B reporting a 1M total is still rejected as too large. (Forward-looking: the
idempotent ingest won't re-render items already in the feed.)

fetch: feed requests now send browser-shaped `Accept` / `Accept-Language`
headers. reqwest sends no Accept by default, which some anti-bot filters 403.
NB this is hygiene, not a Reddit fix — Reddit fingerprints the TLS/HTTP client
(rustls) and hard-403s it where curl only gets rate-limited (429); headers don't
change the fingerprint. The control path (github releases.atom, etc.) is
unaffected.

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

crates/newsfeed-modelwatch/Cargo.toml

@@ -24,6 +24,7 @@ reqwest.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 chrono.workspace = true
+regex.workspace = true
 
 anyhow.workspace = true
 tracing.workspace = true