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/session_suggested_fix.py

@@ -0,0 +1,80 @@
+"""Session suggested-fix model — AI-proposed resolution path for a session.
+
+A session can have multiple suggested fixes over its lifetime as the AI's
+understanding evolves. Only one is active at a time (superseded_at IS NULL);
+emitting a new [SUGGEST_FIX] marker supersedes the prior active one.
+"""
+import uuid
+from datetime import datetime, timezone
+from typing import Any, TYPE_CHECKING
+
+from sqlalchemy import (
+    Text, DateTime, ForeignKey, String, Integer, CheckConstraint,
+)
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+
+from app.core.database import Base
+
+if TYPE_CHECKING:
+    from app.models.ai_session import AISession
+    from app.models.account import Account
+    from app.models.script_template import ScriptTemplate
+
+
+class SessionSuggestedFix(Base):
+    """One AI-proposed fix for a FlowPilot session."""
+    __tablename__ = "session_suggested_fixes"
+    __table_args__ = (
+        CheckConstraint(
+            "confidence_pct BETWEEN 0 AND 100",
+            name="ck_session_suggested_fixes_confidence_pct",
+        ),
+        CheckConstraint(
+            "user_decision IS NULL OR user_decision IN ("
+            "'one_off', 'draft_template', 'build_template', 'dismissed')",
+            name="ck_session_suggested_fixes_user_decision",
+        ),
+    )
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    session_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("ai_sessions.id", ondelete="CASCADE"),
+        nullable=False,
+    )
+    account_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("accounts.id"),
+        nullable=False,
+    )
+    title: Mapped[str] = mapped_column(String(200), nullable=False)
+    description: Mapped[str] = mapped_column(Text, nullable=False)
+    confidence_pct: Mapped[int] = mapped_column(Integer, nullable=False)
+    script_template_id: Mapped[uuid.UUID | None] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("script_templates.id"),
+        nullable=True,
+    )
+    # Populated only when there's no matching template and the AI has
+    # drafted a session-specific script.
+    ai_drafted_script: Mapped[str | None] = mapped_column(Text, nullable=True)
+    ai_drafted_parameters: Mapped[dict[str, Any] | None] = mapped_column(
+        JSONB, nullable=True
+    )
+    user_decision: Mapped[str | None] = mapped_column(String(32), nullable=True)
+    # Set when a newer suggested fix supersedes this one.
+    superseded_at: Mapped[datetime | None] = mapped_column(
+        DateTime(timezone=True), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), default=lambda: datetime.now(timezone.utc)
+    )
+
+    session: Mapped["AISession"] = relationship("AISession", foreign_keys=[session_id])
+    account: Mapped["Account"] = relationship("Account", foreign_keys=[account_id])
+    script_template: Mapped["ScriptTemplate | None"] = relationship(
+        "ScriptTemplate", foreign_keys=[script_template_id]
+    )