Fix #214: ChangeProposal (propose-then-confirm)

Implements non-negotiable #1: the AI assistant never writes autonomously. Every
assistant/contributor "write" emits a ChangeProposal — a structured diff a human
approves, edits, or rejects. Design: docs/design/change-proposal.md.

Structural guarantee: a proposal's operations reach the DB ONLY via
change_proposal_service.apply(), which requires the actor be an editor and
dispatches each op through the normal editing services (person/name/event/
relationship/source/citation create/update/delete) — so every change passes the
privacy engine and is audited as the approving human. propose() only inserts a
pending row; it performs no domain mutation. Model providers stay read-only, so
no model response can mutate tree data.

- ChangeProposal model + migration (status pending|applied|rejected, origin
  assistant|contributor, JSONB operations, reviewer + apply_error).
- Service: propose / list / get / apply (with optional edited ops) / reject /
  delete; a dispatcher mapping ops → editing services. v1 applies ops in order,
  not cross-op transactional (single-op is atomic; documented).
- API /trees/{id}/proposals + a frontend review page (approve/reject; editor-
  gated) and sidebar entry.

Tests: proposal doesn't apply until approved; reject doesn't apply; non-editor
member can see but not apply; multi-op; approve-with-edits; apply-error keeps it
pending. Full suite 87 passed; single alembic head.

Closes #214

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Signed-off-by: Justin Paul <justin@jpaul.me>

backend/app/models/__init__.py

@@ -4,6 +4,7 @@ and for ``create_all`` in tests."""
 from app.models.audit import AuditEntry
 from app.models.auth import Session, UserToken
 from app.models.base import Base
+from app.models.change_proposal import ChangeProposal
 from app.models.event import Event
 from app.models.media import Media
 from app.models.person import Name, Person
@@ -30,4 +31,5 @@ __all__ = [
     "Session",
     "UserToken",
     "Media",
+    "ChangeProposal",
 ]