feat(handoff): email engineer-or-admin teammates on escalation

First half of the Escalation Mode notification dual-path. WebSocket/SSE
push is the second half (next commit) — email handles offline seniors,
push handles online ones for the magic-moment demo.

HandoffManager.dispatch_escalation_notifications:
- Pulls active engineer/admin/owner-role users in the same account_id
  (excludes the escalator + viewers + soft-deleted)
- Sends via existing EmailService.send_notification_email, concurrent
  via asyncio.gather; per-message failures don't block the rest
- Wrapped in try/except: any exception is logged + swallowed. Handoff
  creation is authoritative; notification is advisory. This is the
  graceful-degradation regression both eng + codex reviews flagged as
  critical (handoff must succeed even if SMTP is down).

Endpoint wiring (POST /ai-sessions/{id}/handoff):
- Dispatch fires AFTER db.commit() — never email about a rolled-back
  handoff. Trust-erosion bug if we got that wrong.
- Only fires for intent=escalate. Park is private to the escalator.

Tests (4 new):
- emails-engineer-recipients-in-account: viewer excluded, escalator
  excluded, only the engineer/admin teammates get the message
- skipped-for-park-intent: park doesn't fan out
- graceful-degradation-when-email-raises: RuntimeError from the email
  service does NOT bubble out of dispatch
- endpoint-dispatches-on-escalate: end-to-end wiring through POST

Per-channel delivery records (replacing the dead `notification_sent`
boolean per Codex correction) is a v1.x story — for now application
logs are the audit trail. See
docs/plans/2026-04-27-escalation-mode-wedge-design.md.

20 tests green across handoff_manager + session_handoffs_api +
flowpilot_analytics_escalations. No regressions.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/services/handoff_manager.py

@@ -4,6 +4,7 @@ Creates handoff snapshots, AI assessments (for escalations), claim workflow,
 and queue queries. Dual-writes to ai_sessions.escalation_package for
 backward compatibility with the existing escalation queue.
 """
+import asyncio
 import logging
 from datetime import datetime, timezone
 from typing import Any
@@ -12,9 +13,12 @@ from uuid import UUID
 from sqlalchemy import select
 from sqlalchemy.ext.asyncio import AsyncSession
 
+from app.core.config import settings
+from app.core.email import EmailService
 from app.models.ai_session import AISession
 from app.models.session_branch import SessionBranch
 from app.models.session_handoff import SessionHandoff
+from app.models.user import User
 
 logger = logging.getLogger(__name__)
 
@@ -87,6 +91,102 @@ class HandoffManager:
         await self.db.flush()
         return handoff
 
+    async def dispatch_escalation_notifications(
+        self, handoff: SessionHandoff
+    ) -> int:
+        """Email engineer-or-admin users in the account about a new escalation.
+
+        Call this AFTER `db.commit()` has succeeded — sending email for a
+        rolled-back handoff is the kind of trust-erosion bug that makes pilot
+        customers stop trusting the tool. Returns the number of recipients
+        successfully emailed (best-effort, not authoritative).
+
+        Failures are logged but never raise: the wedge demo's reliability
+        story is "handoff creation always succeeds; notification is best-effort,"
+        not "handoff creation depends on the email service being up." This is
+        the graceful-degradation regression the eng + codex reviews both
+        flagged as critical.
+
+        Per-channel delivery records (Codex correction on the dead
+        `notification_sent` boolean) are a v1.x story — for now the
+        application logs are the audit trail.
+        """
+        if handoff.intent != "escalate":
+            return 0
+
+        try:
+            recipients = (
+                await self.db.execute(
+                    select(User).where(
+                        User.account_id == handoff.account_id,
+                        User.id != handoff.handed_off_by,
+                        User.account_role.in_(("owner", "admin", "engineer")),
+                        User.is_active.is_(True),
+                        User.deleted_at.is_(None),
+                    )
+                )
+            ).scalars().all()
+
+            if not recipients:
+                logger.info(
+                    "No notification recipients for handoff %s in account %s",
+                    handoff.id,
+                    handoff.account_id,
+                )
+                return 0
+
+            # Pull session for the email subject. Fall back to a generic title
+            # if the session is gone (e.g. cascade delete mid-dispatch).
+            session_result = await self.db.execute(
+                select(AISession).where(AISession.id == handoff.session_id)
+            )
+            session = session_result.scalar_one_or_none()
+            problem = (
+                session.problem_summary if session and session.problem_summary
+                else "an active session"
+            )
+
+            title = f"New escalation: {problem}"
+            notes = (handoff.engineer_notes or "").strip()
+            body = (
+                "A teammate has escalated a session and is asking for help.\n\n"
+                f"Reason: {notes if notes else 'No reason provided.'}\n"
+                f"Priority: {handoff.priority}"
+            )
+            link_url = (
+                f"{settings.FRONTEND_URL.rstrip('/')}/escalations"
+                if settings.FRONTEND_URL
+                else None
+            )
+
+            results = await asyncio.gather(
+                *[
+                    EmailService.send_notification_email(
+                        to_email=r.email,
+                        title=title,
+                        body=body,
+                        link_url=link_url,
+                    )
+                    for r in recipients
+                ],
+                return_exceptions=True,
+            )
+            sent = sum(1 for r in results if r is True)
+            logger.info(
+                "Escalation notifications dispatched for handoff %s: %d/%d recipients",
+                handoff.id,
+                sent,
+                len(recipients),
+            )
+            return sent
+
+        except Exception:
+            logger.exception(
+                "Escalation notification dispatch failed for handoff %s",
+                handoff.id,
+            )
+            return 0
+
     async def _generate_snapshot(self, session: AISession) -> dict[str, Any]:
         """Generate a snapshot of the session state at handoff time."""
         snapshot: dict[str, Any] = {