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/schemas/session.py

@@ -55,6 +55,15 @@ class SessionUpdate(BaseModel):
     session_variables: Optional[dict[str, str]] = None
 
 
+class PrepareSessionRequest(BaseModel):
+    """Create a prepared session with pre-filled variables and optional assignee."""
+    tree_id: UUID
+    session_variables: Optional[dict[str, str]] = Field(None, description="Pre-filled intake form values")
+    assigned_to_id: Optional[UUID] = Field(None, description="User ID of the engineer to assign this session to")
+    ticket_number: Optional[str] = Field(None, max_length=100)
+    client_name: Optional[str] = Field(None, max_length=255)
+
+
 class SessionResponse(BaseModel):
     id: UUID
     tree_id: UUID
@@ -63,7 +72,7 @@ class SessionResponse(BaseModel):
     path_taken: list[str]
     decisions: list[dict[str, Any]]
     custom_steps: list[dict[str, Any]] = Field(default_factory=list)
-    started_at: datetime
+    started_at: Optional[datetime] = None
     completed_at: Optional[datetime] = None
     outcome: Optional[SessionOutcome] = None
     outcome_notes: Optional[str] = None
@@ -74,6 +83,10 @@ class SessionResponse(BaseModel):
     scratchpad: str = ""
     session_variables: dict[str, str] = Field(default_factory=dict)
 
+    # Prepared session fields
+    prepared_by_id: Optional[UUID] = None
+    assigned_to_id: Optional[UUID] = None
+
     @validator('scratchpad', 'next_steps', pre=True, always=True)
     def normalize_text_fields(cls, v):
         return v or ""
@@ -106,6 +119,11 @@ class SessionComplete(BaseModel):
     next_steps: Optional[str] = None
 
 
+class SessionVariablesUpdate(BaseModel):
+    """Partial update to session variables (dict merge)."""
+    variables: dict[str, str] = Field(..., description="Key-value pairs to merge into session_variables")
+
+
 class ScratchpadUpdate(BaseModel):
     scratchpad: str