Phase 2: Add reasoning and TipTap content endpoints

- Extend Document model with reasoning_type, confidence, reasoning_steps, model_source, tiptap_content fields
- Add new endpoints:
  - GET /documents/{id}/reasoning - Get reasoning metadata
  - PATCH /documents/{id}/reasoning - Update reasoning metadata
  - GET /documents/{id}/reasoning-panel - Get reasoning panel data for UI
  - POST /documents/{id}/reasoning-steps - Add reasoning step
  - DELETE /documents/{id}/reasoning-steps/{step} - Delete reasoning step
  - GET /documents/{id}/content?format=tiptap|markdown - Get content in TipTap or Markdown
  - PUT /documents/{id}/content - Update content (supports both TipTap JSON and Markdown)
- Add TipTap to Markdown conversion
- Update database schema with new columns
- Add comprehensive tests for all new endpoints
- All 37 tests passing

app/models/document.py

@@ -1,5 +1,6 @@
 import uuid
 from datetime import datetime
+from enum import Enum
 
 from sqlalchemy import Boolean, DateTime, ForeignKey, String, Text
 from sqlalchemy.orm import Mapped, mapped_column, relationship
@@ -11,6 +12,13 @@ def generate_uuid() -> str:
     return str(uuid.uuid4())
 
 
+class ReasoningType(str, Enum):
+    CHAIN = "chain"
+    IDEA = "idea"
+    CONTEXT = "context"
+    REFLECTION = "reflection"
+
+
 class Document(Base):
     __tablename__ = "documents"
 
@@ -25,3 +33,9 @@ class Document(Base):
     deleted_by: Mapped[str | None] = mapped_column(String(36), nullable=True)
     created_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, nullable=False)
     updated_at: Mapped[datetime] = mapped_column(DateTime, default=datetime.utcnow, onupdate=datetime.utcnow, nullable=False)
+    # Phase 2: Reasoning fields
+    reasoning_type: Mapped[str | None] = mapped_column(String(20), nullable=True)
+    confidence: Mapped[str | None] = mapped_column(String(10), nullable=True)  # Store as string to handle JSON serialization
+    reasoning_steps: Mapped[str | None] = mapped_column(Text, nullable=True)  # JSON array as text
+    model_source: Mapped[str | None] = mapped_column(String(100), nullable=True)
+    tiptap_content: Mapped[str | None] = mapped_column(Text, nullable=True)  # JSON object as text