Add sources and citations API (Phase 1: sources-first spine)

Source CRUD (reusable, tree-scoped) and Citation create/list/soft-delete linking one source to exactly one fact (person/event/name/relationship). Editor-gated writes, privacy-filtered reads, audit throughout; tenant + existence validation on source and target. list_citations returns all tree citations so the UI can render 'sourced' indicators in one round-trip. 22 tests pass.

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

backend/app/api/v1/__init__.py

@@ -2,7 +2,16 @@
 
 from fastapi import APIRouter
 
-from app.api.v1 import auth, events, persons, relationships, trees, users
+from app.api.v1 import (
+    auth,
+    citations,
+    events,
+    persons,
+    relationships,
+    sources,
+    trees,
+    users,
+)
 
 api_router = APIRouter(prefix="/api/v1")
 api_router.include_router(auth.router)
@@ -11,3 +20,5 @@ api_router.include_router(trees.router)
 api_router.include_router(persons.router)
 api_router.include_router(events.router)
 api_router.include_router(relationships.router)
+api_router.include_router(sources.router)
+api_router.include_router(citations.router)