feat: add cockpit triage backend foundation (Phase 1)

- Migration 071: add client_name, asset_name, issue_category,
  triage_hypothesis, evidence_items columns to ai_sessions
- TriageUpdate schema for AI-inferred header updates in chat responses
- QuestionItem.options field for quick-reply buttons
- PATCH /ai-sessions/{id}/triage endpoint for manual header edits
- POST /ai-sessions/{id}/handoff-draft streaming endpoint for conclude modal
- Structured handoff fields (root_cause, steps_taken, recommendations)
  on resolve/escalate requests, passed through to ResolutionOutputGenerator
- Triage fields exposed in AISessionDetail response for session resume

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

backend/app/models/ai_session.py

@@ -137,6 +137,28 @@ class AISession(Base):
         comment="Snapshot of PSA ticket data at session start",
     )
 
+    # ── Triage / Cockpit Header ──
+    client_name: Mapped[Optional[str]] = mapped_column(
+        String(255), nullable=True,
+        comment="MSP client name for incident header (AI-inferred or manual)",
+    )
+    asset_name: Mapped[Optional[str]] = mapped_column(
+        String(255), nullable=True,
+        comment="Device, asset, or user being worked on",
+    )
+    issue_category: Mapped[Optional[str]] = mapped_column(
+        String(100), nullable=True,
+        comment="Human-readable category (e.g. DNS / Networking)",
+    )
+    triage_hypothesis: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True,
+        comment="Current working hypothesis — AI-updated + engineer-editable",
+    )
+    evidence_items: Mapped[Optional[list[dict[str, Any]]]] = mapped_column(
+        JSONB, nullable=True,
+        comment='What We Know list: [{"text": str, "status": "confirmed"|"ruled_out"|"pending"}]',
+    )
+
     # ── Resolution / Escalation ──
     resolution_summary: Mapped[Optional[str]] = mapped_column(
         Text, nullable=True,