feat: flexible intake — deferred variables + prepared sessions

Remove blocking intake form modal. Variables are now filled inline during
flow execution or pre-filled via prepared sessions. Adds PATCH /sessions/{id}/variables
endpoint, POST /sessions/prepare for session pre-staging, inline variable prompts
in StepDetail, editable Session Variables panel, and "Prepared for You" dashboard section.

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

backend/app/models/user.py

@@ -99,7 +99,7 @@ class User(Base):
     owned_account: Mapped[Optional["Account"]] = relationship("Account", foreign_keys="[Account.owner_id]", back_populates="owner", uselist=False)
     team: Mapped[Optional["Team"]] = relationship("Team", back_populates="users")
     trees: Mapped[list["Tree"]] = relationship("Tree", foreign_keys="[Tree.author_id]", back_populates="author")
-    sessions: Mapped[list["Session"]] = relationship("Session", back_populates="user")
+    sessions: Mapped[list["Session"]] = relationship("Session", foreign_keys="[Session.user_id]", back_populates="user")
     folders: Mapped[list["UserFolder"]] = relationship("UserFolder", back_populates="user")
 
     @property