feat(models): Phase 1 SQLAlchemy models — SessionFact, SessionSuggestedFix, DraftTemplate, AccountSettings

Backs the schema added in 210d310 with SQLAlchemy 2.0 models.

- SessionFact: "What we know" facts with polymorphic source_ref pointing
  at task-lane item UUIDs inside ai_sessions.pending_task_lane (not a FK
  per Section 4.2).
- SessionSuggestedFix: AI-proposed resolutions with supersession tracking
  and the full user_decision state machine.
- DraftTemplate: post-resolve templatization queue with promotion to
  script_templates.
- AccountSettings: per-account JSONB preferences grab-bag with async
  classmethod helpers — get_setting(db, account_id, key, default) reads
  without creating, set_setting(db, account_id, key, value) upserts via
  Postgres ON CONFLICT + jsonb `||` merge so existing keys are preserved.
  Lazy row creation matches the Phase 1 design.

Column additions on existing models to mirror the migration:
- AISession: resolution_note_* / escalation_package_* / state_version
  (the preview-cache-invalidation counter consumed by Phase 3).
- ScriptTemplate: source_session_id / source_user_id / source_ticket_ref
  (provenance for templates promoted from DraftTemplate).

All four new models registered in app.models.__init__ and __all__.
TYPE_CHECKING-guarded relationship imports throughout, matching the
repo's existing model style.

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

backend/app/models/ai_session.py

@@ -214,6 +214,38 @@ class AISession(Base):
         comment="Current task lane state: {questions: [...], actions: [...]}",
     )
 
+    # ── Resolution / Escalation artifacts (Phase 1 — FlowPilot migration) ──
+    # Markdown of the posted note + PSA external ID for round-trip traceability.
+    resolution_note_markdown: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True,
+        comment="Final Resolve note markdown, as posted to the PSA",
+    )
+    resolution_note_posted_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True,
+    )
+    resolution_note_external_id: Mapped[Optional[str]] = mapped_column(
+        String(128), nullable=True,
+        comment="PSA (e.g. CW) ticket-note ID returned at post time",
+    )
+    escalation_package_markdown: Mapped[Optional[str]] = mapped_column(
+        Text, nullable=True,
+        comment="Final Escalate handoff package markdown, as posted to the PSA",
+    )
+    escalation_package_posted_at: Mapped[Optional[datetime]] = mapped_column(
+        DateTime(timezone=True), nullable=True,
+    )
+    escalation_package_external_id: Mapped[Optional[str]] = mapped_column(
+        String(128), nullable=True,
+        comment="PSA ticket-note ID for the escalation package",
+    )
+    # Incremented atomically by any write that invalidates the resolution
+    # note preview cache (facts, suggested fixes, script generations).
+    # See FLOWPILOT-MIGRATION.md Section 5.5.
+    state_version: Mapped[int] = mapped_column(
+        Integer, nullable=False, default=0, server_default=sa.text("0"),
+        comment="Monotonic preview-cache version; bumped on state-changing writes",
+    )
+
     # ── Branching ──
     is_branching: Mapped[bool] = mapped_column(
         default=False,