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/sources.py

@@ -0,0 +1,48 @@
+import uuid
+
+from fastapi import APIRouter, status
+
+from app.api.deps import CurrentUser, SessionDep
+from app.schemas.source import SourceCreate, SourceRead
+from app.services import source_service, tree_service
+
+router = APIRouter(prefix="/trees", tags=["sources"])
+
+
+@router.post("/{tree_id}/sources", response_model=SourceRead, status_code=status.HTTP_201_CREATED)
+async def create_source(
+    tree_id: uuid.UUID, data: SourceCreate, session: SessionDep, current: CurrentUser
+) -> SourceRead:
+    tree = await tree_service.get_tree(session, viewer_id=current.id, tree_id=tree_id)
+    source = await source_service.create_source(
+        session, actor=current, tree=tree, **data.model_dump()
+    )
+    return SourceRead.model_validate(source)
+
+
+@router.get("/{tree_id}/sources", response_model=list[SourceRead])
+async def list_sources(
+    tree_id: uuid.UUID, session: SessionDep, current: CurrentUser
+) -> list[SourceRead]:
+    tree = await tree_service.get_tree(session, viewer_id=current.id, tree_id=tree_id)
+    sources = await source_service.list_sources(session, viewer_id=current.id, tree=tree)
+    return [SourceRead.model_validate(s) for s in sources]
+
+
+@router.get("/{tree_id}/sources/{source_id}", response_model=SourceRead)
+async def get_source(
+    tree_id: uuid.UUID, source_id: uuid.UUID, session: SessionDep, current: CurrentUser
+) -> SourceRead:
+    tree = await tree_service.get_tree(session, viewer_id=current.id, tree_id=tree_id)
+    source = await source_service.get_source(
+        session, viewer_id=current.id, tree=tree, source_id=source_id
+    )
+    return SourceRead.model_validate(source)
+
+
+@router.delete("/{tree_id}/sources/{source_id}", status_code=status.HTTP_204_NO_CONTENT)
+async def delete_source(
+    tree_id: uuid.UUID, source_id: uuid.UUID, session: SessionDep, current: CurrentUser
+) -> None:
+    tree = await tree_service.get_tree(session, viewer_id=current.id, tree_id=tree_id)
+    await source_service.delete_source(session, actor=current, tree=tree, source_id=source_id)