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/__init__.py

@@ -58,6 +58,10 @@ from .template_tree import TemplateTree
 from .platform_step import PlatformStep
 from .device_type import DeviceType
 from .network_diagram import NetworkDiagram
+from .session_fact import SessionFact
+from .session_suggested_fix import SessionSuggestedFix
+from .draft_template import DraftTemplate
+from .account_settings import AccountSettings
 
 __all__ = [
     "User",
@@ -130,4 +134,8 @@ __all__ = [
     "PlatformStep",
     "DeviceType",
     "NetworkDiagram",
+    "SessionFact",
+    "SessionSuggestedFix",
+    "DraftTemplate",
+    "AccountSettings",
 ]