feat: add account management, email verification, AI fixes, and user guides

- Profile settings, account transfer, delete/leave account flows
- Email verification with JWT tokens and Resend integration
- AI assistant/copilot fixes: markdown rendering, shared RAG helpers,
  token tracking, input refocus, model_validate usage
- User guides hub + detail pages with 13 topic guides
- Sidebar and top bar navigation for guides

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/models/user.py

@@ -68,6 +68,15 @@ class User(Base):
     )
     last_login: Mapped[Optional[datetime]] = mapped_column(DateTime(timezone=True), nullable=True)
 
+    # Profile fields
+    phone: Mapped[Optional[str]] = mapped_column(String(50), nullable=True)
+    job_title: Mapped[Optional[str]] = mapped_column(String(255), nullable=True)
+    timezone: Mapped[str] = mapped_column(String(100), nullable=False, default="UTC", server_default="UTC")
+    avatar_url: Mapped[Optional[str]] = mapped_column(String(500), nullable=True)
+    email_verified_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+
     # AI billing cycle anchor (for quota reset calculation)
     ai_billing_cycle_anchor_at: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True), nullable=True