feat(l1): extend FlowProposal with source/linked_ticket/validated_by_outcome

Adds source (NOT NULL, backfilled to 'manual_draft'), linked_ticket_id,
linked_ticket_kind, validated_by_outcome columns. CHECK constraints on
source values and linked_ticket_kind values. walked_path lives on the
new l1_walk_sessions table (Task 6) — NOT on FlowProposal.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/models/flow_proposal.py

@@ -7,7 +7,7 @@ import uuid
 from datetime import datetime, timezone
 from typing import Optional, Any, TYPE_CHECKING
 
-from sqlalchemy import String, Text, DateTime, ForeignKey, Integer, Float, CheckConstraint
+from sqlalchemy import String, Text, DateTime, ForeignKey, Integer, Float, Boolean, CheckConstraint, text as sa_text
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 from sqlalchemy.dialects.postgresql import UUID, JSONB
 
@@ -48,6 +48,14 @@ class FlowProposal(Base):
             "status IN ('pending', 'approved', 'modified', 'rejected', 'dismissed', 'auto_reinforced')",
             name="ck_flow_proposals_status",
         ),
+        CheckConstraint(
+            "source IN ('ai_realtime_l1', 'kb_accelerator', 'manual_draft', 'ai_promoted')",
+            name="ck_flow_proposals_source",
+        ),
+        CheckConstraint(
+            "linked_ticket_kind IS NULL OR linked_ticket_kind IN ('psa', 'internal')",
+            name="ck_flow_proposals_linked_ticket_kind",
+        ),
     )
 
     id: Mapped[uuid.UUID] = mapped_column(
@@ -135,6 +143,16 @@ class FlowProposal(Base):
         comment="The flow that was created/updated when this proposal was approved",
     )
 
+    # ── L1 workspace ──
+    source: Mapped[str] = mapped_column(
+        String(30), nullable=False, server_default=sa_text("'manual_draft'"),
+    )
+    linked_ticket_id: Mapped[Optional[str]] = mapped_column(String(64), nullable=True)
+    linked_ticket_kind: Mapped[Optional[str]] = mapped_column(String(10), nullable=True)
+    validated_by_outcome: Mapped[bool] = mapped_column(
+        Boolean(), nullable=False, server_default=sa_text('false'),
+    )
+
     # ── Timestamps ──
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)