fix(db): widen ai_sessions.status column from varchar(20) to varchar(30)

'requesting_escalation' is 23 characters, exceeding the varchar(20)
limit. This caused a StringDataRightTruncationError 500 on escalate.

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

backend/app/models/ai_session.py

@@ -85,7 +85,7 @@ class AISession(Base):
 
     # ── Session state ──
     status: Mapped[str] = mapped_column(
-        String(20), nullable=False, default="active", index=True,
+        String(30), nullable=False, default="active", index=True,
     )
     confidence_tier: Mapped[str] = mapped_column(
         String(20), nullable=False, default="discovery",