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/session.py

@@ -35,8 +35,9 @@ class Session(Base):
     path_taken: Mapped[list[str]] = mapped_column(JSONB, nullable=False, default=list)
     decisions: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list)
     custom_steps: Mapped[list[dict[str, Any]]] = mapped_column(JSONB, nullable=False, default=list)
-    started_at: Mapped[datetime] = mapped_column(
+    started_at: Mapped[Optional[datetime]] = mapped_column(
         DateTime(timezone=True),
+        nullable=True,
         default=lambda: datetime.now(timezone.utc),
         index=True
     )
@@ -60,9 +61,25 @@ class Session(Base):
         Text, nullable=True, server_default=sa.text("''")
     )
 
+    # Prepared sessions (flexible intake)
+    prepared_by_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=True,
+        index=True
+    )
+    assigned_to_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=True,
+        index=True
+    )
+
     # Relationships
     tree: Mapped["Tree"] = relationship("Tree", back_populates="sessions")
-    user: Mapped["User"] = relationship("User", back_populates="sessions")
+    user: Mapped["User"] = relationship("User", foreign_keys=[user_id], back_populates="sessions")
+    prepared_by: Mapped[Optional["User"]] = relationship("User", foreign_keys=[prepared_by_id])
+    assigned_to: Mapped[Optional["User"]] = relationship("User", foreign_keys=[assigned_to_id])
     attachments: Mapped[list["Attachment"]] = relationship("Attachment", back_populates="session")
     shares: Mapped[list["SessionShare"]] = relationship("SessionShare", back_populates="session", cascade="all, delete-orphan")
 

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