v0.24.0: Claude Haiku tag suggestions on propose-RFC (roadmap #27)

The §9.1 Slice-2 AI-suggested tag chips, deferred since the propose
modal landed, now wired up. As the propose-RFC draft fills in, the
backend asks Claude Haiku for tags drawn ONLY from the corpus's
existing tag set (the de-facto taxonomy — v1 tags are free-form chip
input, there is no curated list), surfaced as clickable suggestion
chips. Nothing auto-applies; the user clicks to add.

Backend:
- tag_suggest.py: universe gather (distinct corpus tags, most-common
  first), Haiku prompt + tolerant reply parser (drops invented tags,
  dedupes, clamps confidence), in-process per-user rate limit.
- providers.construct_haiku(): dedicated Haiku provider from the
  operator key, independent of ENABLED_MODELS — tag suggestion always
  uses the cheap+fast model. No RFC slug at propose time, so the §6.7
  funder path does not apply.
- POST /api/rfcs/suggest-tags: contributor-gated, rate-limited.
  Degrades to an empty list (never an error) when no Anthropic key is
  bound, the corpus has no tags, or the draft is empty.

Frontend:
- ProposeModal: debounced suggestion fetch (700ms, stale-response
  guarded), clickable suggestion chips, and the required inline
  disclosure that the draft text is sent to Anthropic.
- api.suggestTags(): forgiving — any non-OK resolves to [].

Tests: 11 new (vertical contributor-gating / filtering / no-key /
empty-corpus / 429, plus units for gather, parser tolerance, max,
short-circuit, provider-failure). Full suite 351 green.

New secret on deploy: ANTHROPIC_API_KEY (see CHANGELOG Upgrade steps).
Disclosure copy wants a counsel pass before deploy, per #22 discipline.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/app/providers.py

@@ -184,6 +184,21 @@ def load_providers(env: dict) -> dict[str, BaseProvider]:
     return providers
 
 
+def construct_haiku(api_key: str) -> AnthropicProvider:
+    """A dedicated Claude Haiku provider, independent of the
+    `ENABLED_MODELS` chat-picker universe.
+
+    The §9.1 tag-suggestion surface (roadmap #27) always wants the
+    cheap + fast model regardless of which models the operator surfaces
+    in the §8.12 picker, so it constructs Haiku directly from the
+    operator's Anthropic key rather than going through `load_providers`.
+    The model id is sourced from the same `_CLAUDE_VARIANTS` table the
+    picker uses, so a model-string bump lands in one place.
+    """
+    model, name = _CLAUDE_VARIANTS["claude-haiku"]
+    return AnthropicProvider(api_key=api_key, model=model, display_name=name)
+
+
 def load_from_config(config) -> dict[str, BaseProvider]:
     """Convenience adapter so callers can pass our Config dataclass directly."""
     env = {