feat: add message reliability foundation

- implement idempotent message creation via client_message_id

- add persistent delivered/read receipts

- expose /messages/status and wire websocket receipt events

- update web client to send client ids and auto-ack delivered/read

app/messages/models.py

@@ -2,7 +2,7 @@ from datetime import datetime
 from enum import Enum
 from typing import TYPE_CHECKING
 
-from sqlalchemy import DateTime, Enum as SAEnum, ForeignKey, Text, func
+from sqlalchemy import DateTime, Enum as SAEnum, ForeignKey, String, Text, UniqueConstraint, func
 from sqlalchemy.orm import Mapped, mapped_column, relationship
 
 from app.database.base import Base
@@ -42,3 +42,34 @@ class Message(Base):
     chat: Mapped["Chat"] = relationship(back_populates="messages")
     sender: Mapped["User"] = relationship(back_populates="sent_messages")
     attachments: Mapped[list["Attachment"]] = relationship(back_populates="message", cascade="all, delete-orphan")
+
+
+class MessageIdempotencyKey(Base):
+    __tablename__ = "message_idempotency_keys"
+    __table_args__ = (
+        UniqueConstraint("chat_id", "sender_id", "client_message_id", name="uq_msg_idem_chat_sender_client"),
+    )
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    chat_id: Mapped[int] = mapped_column(ForeignKey("chats.id", ondelete="CASCADE"), nullable=False, index=True)
+    sender_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    client_message_id: Mapped[str] = mapped_column(String(64), nullable=False)
+    message_id: Mapped[int] = mapped_column(ForeignKey("messages.id", ondelete="CASCADE"), nullable=False, index=True)
+    created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), server_default=func.now(), nullable=False)
+
+
+class MessageReceipt(Base):
+    __tablename__ = "message_receipts"
+    __table_args__ = (UniqueConstraint("chat_id", "user_id", name="uq_msg_receipts_chat_user"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True, index=True)
+    chat_id: Mapped[int] = mapped_column(ForeignKey("chats.id", ondelete="CASCADE"), nullable=False, index=True)
+    user_id: Mapped[int] = mapped_column(ForeignKey("users.id", ondelete="CASCADE"), nullable=False, index=True)
+    last_delivered_message_id: Mapped[int | None] = mapped_column(nullable=True)
+    last_read_message_id: Mapped[int | None] = mapped_column(nullable=True)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )