feat(phase-9): email dispatcher — HTML templates, delivery log, test endpoint

Backend:
- app/models/email_log.py: EmailLog table (school_id, to, subject, type, status,
  error, sent_at) with EmailType + EmailStatus enums
- migrations/002_phase9_email_logs.py: Alembic migration for email_logs table
- app/templates/email/: 6 Jinja2 HTML templates — base layout, invoice,
  low_credit, license_expiry, overdue_warning, suspension
- app/services/email.py: enhanced send_email() — accepts template_name+context
  for HTML rendering, logs every attempt to email_logs, retries up to 3x on
  transient SMTP failure with exponential backoff
- app/routers/email.py: GET /api/email/logs (paginated, filterable by type/status/school),
  POST /api/email/test (send test email, super admin)
- tasks/billing.py: invoice + overdue warning + suspension emails now use HTML templates
- tasks/sms.py: low credit alert now uses HTML template
- tasks/license.py: expiry warning now uses HTML template
- app/main.py + migrations/env.py: wire in email_log model + email router

Frontend:
- EmailLogsPage.vue: table with to/subject/type badge/status badge/sent_at/error,
  type+status filters, pagination, Send Test Email modal
- router/index.ts: /email-logs route
- AppSidebar.vue: Email Logs nav item
- api.ts: getEmailLogs, sendTestEmail

backend/app/models/email_log.py

@@ -0,0 +1,42 @@
+"""Email delivery log model."""
+import uuid
+import enum
+from datetime import datetime, timezone
+from sqlalchemy import String, DateTime, Enum as SAEnum, ForeignKey, Text
+from sqlalchemy.orm import Mapped, mapped_column
+from app.database import Base
+
+
+class EmailStatus(str, enum.Enum):
+    sent = "sent"
+    failed = "failed"
+
+
+class EmailType(str, enum.Enum):
+    invoice = "invoice"
+    low_credit = "low_credit"
+    license_expiry = "license_expiry"
+    overdue_warning = "overdue_warning"
+    suspension = "suspension"
+    monthly_report = "monthly_report"
+    welcome = "welcome"
+    test = "test"
+    other = "other"
+
+
+class EmailLog(Base):
+    __tablename__ = "email_logs"
+
+    id: Mapped[str] = mapped_column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    school_id: Mapped[str | None] = mapped_column(
+        String(36), ForeignKey("schools.id", ondelete="SET NULL"), nullable=True, index=True
+    )
+    to_email: Mapped[str] = mapped_column(String(255), nullable=False, index=True)
+    subject: Mapped[str] = mapped_column(String(500), nullable=False)
+    email_type: Mapped[EmailType] = mapped_column(SAEnum(EmailType), default=EmailType.other, nullable=False, index=True)
+    status: Mapped[EmailStatus] = mapped_column(SAEnum(EmailStatus), nullable=False, index=True)
+    error_message: Mapped[str | None] = mapped_column(Text, nullable=True)
+    sent_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)
+    )