feat: add audit log table and integration with admin/tree endpoints

Creates AuditLog model with JSONB details column for tracking admin
actions. Integrates log_audit() helper into admin endpoints (role
change, team admin toggle, deactivate, activate) and tree delete.
IP address column reserved for future Railway proxy header support.

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

backend/app/models/audit_log.py

@@ -0,0 +1,35 @@
+import uuid
+from datetime import datetime, timezone
+from typing import Optional
+from sqlalchemy import String, DateTime, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column
+from sqlalchemy.dialects.postgresql import UUID, JSONB
+from app.core.database import Base
+
+
+class AuditLog(Base):
+    __tablename__ = "audit_logs"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        primary_key=True,
+        default=uuid.uuid4
+    )
+    user_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id"),
+        nullable=False,
+        index=True
+    )
+    action: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    resource_type: Mapped[str] = mapped_column(String(50), nullable=False, index=True)
+    resource_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        nullable=True
+    )
+    details: Mapped[Optional[dict]] = mapped_column(JSONB, nullable=True)
+    ip_address: Mapped[Optional[str]] = mapped_column(String(45), nullable=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc)
+    )