feat(pilot): add outcome tracking columns to session_suggested_fixes

Phase 8 prep for the fix outcome banner. Adds:
- status (proposed|applied_success|applied_failed|applied_partial|dismissed)
- applied_at, verified_at (timestamps)
- partial_notes, failure_reason (engineer-provided context)
- ai_outcome_proposal (JSONB for AI [FIX_OUTCOME] marker payloads)

Backfills status='dismissed' from user_decision='dismissed'. status is
orthogonal to user_decision — outcome (did the fix work?) vs script-path.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/models/session_suggested_fix.py

@@ -35,6 +35,11 @@ class SessionSuggestedFix(Base):
             "'one_off', 'draft_template', 'build_template', 'dismissed')",
             name="ck_session_suggested_fixes_user_decision",
         ),
+        CheckConstraint(
+            "status IN ('proposed', 'applied_success', 'applied_failed', "
+            "'applied_partial', 'dismissed')",
+            name="ck_session_suggested_fixes_status",
+        ),
     )
 
     id: Mapped[uuid.UUID] = mapped_column(
@@ -65,6 +70,21 @@ class SessionSuggestedFix(Base):
         JSONB, nullable=True
     )
     user_decision: Mapped[str | None] = mapped_column(String(32), nullable=True)
+    # Outcome dimension — did the fix work? Orthogonal to user_decision.
+    status: Mapped[str] = mapped_column(
+        String(20), nullable=False, default="proposed"
+    )
+    applied_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    verified_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    partial_notes: Mapped[str | None] = mapped_column(Text, nullable=True)
+    failure_reason: Mapped[str | None] = mapped_column(Text, nullable=True)
+    ai_outcome_proposal: Mapped[dict[str, Any] | None] = mapped_column(
+        JSONB, nullable=True
+    )
     # Set when a newer suggested fix supersedes this one.
     superseded_at: Mapped[datetime | None] = mapped_column(
         DateTime(timezone=True), nullable=True