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/schemas/source.py

@@ -0,0 +1,61 @@
+import uuid
+from datetime import datetime
+
+from pydantic import BaseModel, ConfigDict
+
+from app.models.enums import CitationConfidence
+
+
+class SourceCreate(BaseModel):
+    title: str
+    author: str | None = None
+    source_type: str | None = None
+    repository: str | None = None
+    url: str | None = None
+    citation_text: str | None = None
+    publication_info: str | None = None
+    quality_note: str | None = None
+
+
+class SourceRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    tree_id: uuid.UUID
+    title: str
+    author: str | None
+    source_type: str | None
+    repository: str | None
+    url: str | None
+    citation_text: str | None
+    publication_info: str | None
+    quality_note: str | None
+    created_at: datetime
+
+
+class CitationCreate(BaseModel):
+    source_id: uuid.UUID
+    # Exactly one target fact.
+    person_id: uuid.UUID | None = None
+    event_id: uuid.UUID | None = None
+    name_id: uuid.UUID | None = None
+    relationship_id: uuid.UUID | None = None
+    page: str | None = None
+    detail: str | None = None
+    confidence: CitationConfidence | None = None
+
+
+class CitationRead(BaseModel):
+    model_config = ConfigDict(from_attributes=True)
+
+    id: uuid.UUID
+    tree_id: uuid.UUID
+    source_id: uuid.UUID
+    person_id: uuid.UUID | None
+    event_id: uuid.UUID | None
+    name_id: uuid.UUID | None
+    relationship_id: uuid.UUID | None
+    page: str | None
+    detail: str | None
+    confidence: CitationConfidence | None
+    created_at: datetime