feat(models): 组织、用户、角色、客户、会话与消息模型

按组织边界定义数据关系，补齐仓储层与通用工具，迁移脚本可重复执行。

backend/app/models/fcm_token.py

@@ -0,0 +1,55 @@
+"""
+Copyright 2024-2026 ChatterMate
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+"""
+
+from sqlalchemy import Column, String, DateTime, ForeignKey, func
+from sqlalchemy.orm import relationship
+from sqlalchemy.dialects.postgresql import UUID
+import uuid
+
+from app.database import Base
+
+
+class FCMToken(Base):
+    """One web-push token per signed-in browser.
+
+    Replaces the old single `users.fcm_token_web` column, which held one token
+    for the whole account: signing in on a laptop overwrote the phone's token
+    and signing out anywhere cleared it for every device, so the phone silently
+    stopped receiving pushes.
+
+    The token is minted by the browser, not by us, and FCM returns the *same*
+    value for a given browser install regardless of who is signed in — hence the
+    unique constraint plus reassign-on-register in the repository, so a shared
+    browser moves its token to the new owner instead of delivering that user's
+    notifications to the previous one.
+    """
+
+    __tablename__ = "fcm_tokens"
+
+    id = Column(UUID(as_uuid=True), primary_key=True, default=uuid.uuid4)
+    user_id = Column(
+        UUID(as_uuid=True),
+        ForeignKey("users.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    token = Column(String, nullable=False, unique=True)
+    created_at = Column(DateTime(timezone=True), server_default=func.now())
+    updated_at = Column(
+        DateTime(timezone=True), server_default=func.now(), onupdate=func.now()
+    )
+
+    user = relationship("User", back_populates="fcm_tokens")