wip(handoff): restore backend suite to green

Co-Authored-By: Codex <noreply@openai.com>

backend/app/models/ai_session.py

@@ -10,7 +10,7 @@ from typing import Optional, Any, TYPE_CHECKING
 from sqlalchemy import String, Text, DateTime, ForeignKey, Boolean, Integer, Float, CheckConstraint
 import sqlalchemy as sa
 from sqlalchemy.orm import Mapped, mapped_column, relationship
-from sqlalchemy.dialects.postgresql import UUID, JSONB
+from sqlalchemy.dialects.postgresql import UUID, JSONB, TSVECTOR
 
 from app.core.database import Base
 
@@ -46,6 +46,7 @@ class AISession(Base):
             "confidence_tier IN ('guided', 'exploring', 'discovery')",
             name="ck_ai_sessions_confidence_tier",
         ),
+        sa.Index("idx_ai_sessions_search", "search_vector", postgresql_using="gin"),
     )
 
     id: Mapped[uuid.UUID] = mapped_column(
@@ -150,6 +151,18 @@ class AISession(Base):
         Text, nullable=True,
         comment="Why escalated (set on escalation)",
     )
+    search_vector: Mapped[Optional[str]] = mapped_column(
+        TSVECTOR,
+        sa.Computed(
+            "to_tsvector('english', "
+            "coalesce(problem_summary, '') || ' ' || "
+            "coalesce(resolution_summary, '') || ' ' || "
+            "coalesce(escalation_reason, '') || ' ' || "
+            "coalesce(problem_domain, ''))",
+            persisted=True,
+        ),
+        nullable=True,
+    )
     escalation_package: Mapped[Optional[dict[str, Any]]] = mapped_column(
         JSONB, nullable=True,
         comment="Context package for receiving engineer: steps_tried, hypotheses, suggestions",