feat(rules): project-scoped rules (S3)

Rules can now belong to either a rulebook topic OR a single project,
enforced by a CHECK constraint (exactly-one of topic_id/project_id).
Adds the create_project_rule MCP tool + REST endpoint, surfaces
project-scoped rules in get_project/get_task/start_planning under a
new project_rules field, and adds a project Rules tab section with an
inline create form so the operator can author project rules from the
UI without rulebook ceremony.

- migration 0059: rules.project_id (FK projects ON DELETE CASCADE),
  topic_id now nullable, CHECK ck_rule_topic_xor_project, index on
  project_id
- model: Rule gains project_id; to_dict exposes it
- service: create_project_rule with project-ownership guard; list_rules
  with project_id filter UNIONs subscription-derived + project-scoped;
  get_applicable_rules adds a project_rules field; get_rule / update_rule
  / delete_rule fetch via a shared _fetch_owned_rule that handles both
  rulebook and project ownership paths
- trash: project delete cascades to project-scoped rules
- MCP: create_project_rule tool registered; _INSTRUCTIONS mentions both
  create_rule and create_project_rule paths
- REST: POST /api/projects/<id>/rules (statement required, title derived
  if omitted)
- frontend: Rule type gains nullable topic_id + project_id; createProjectRule
  client; ProjectRulesTab.vue gains a "Project rules" section with inline
  create form and per-rule expand/delete
- tests: register count → 18; create_project_rule unit tests (required
  fields, title derivation, explicit-title pass-through); applicable_rules
  shape tests now include project_rules; trash cascade test updated to
  expect 5 executions

S1+S2 (always_on flag + Scribe-first prompt) shipped in 658348f.
S4 (enter_project handshake) follows.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/fabledassistant/services/trash.py

@@ -53,6 +53,8 @@ async def _cascade(session, user_id: int, etype: str, eid: int, batch: str, now)
     if etype == "project":
         await _set(session, Note, [Note.user_id == user_id, Note.project_id == eid], batch, now)
         await _set(session, Milestone, [Milestone.user_id == user_id, Milestone.project_id == eid], batch, now)
+        # Project-scoped rules cascade with the project they're attached to.
+        await _set(session, Rule, [Rule.project_id == eid], batch, now)
         await _set(session, Project, [Project.user_id == user_id, Project.id == eid], batch, now)
     elif etype == "milestone":
         await _set(session, Note, [Note.user_id == user_id, Note.milestone_id == eid], batch, now)