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

alembic/versions/0002_message_reliability_tables.py

@@ -0,0 +1,83 @@
+"""message reliability tables
+
+Revision ID: 0002_message_reliability_tables
+Revises: 0001_initial_schema
+Create Date: 2026-03-08 01:30:00.000000
+"""
+
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+
+revision: str = "0002_message_reliability_tables"
+down_revision: Union[str, Sequence[str], None] = "0001_initial_schema"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "message_idempotency_keys",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("chat_id", sa.Integer(), nullable=False),
+        sa.Column("sender_id", sa.Integer(), nullable=False),
+        sa.Column("client_message_id", sa.String(length=64), nullable=False),
+        sa.Column("message_id", sa.Integer(), nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.ForeignKeyConstraint(["chat_id"], ["chats.id"], name=op.f("fk_message_idempotency_keys_chat_id_chats"), ondelete="CASCADE"),
+        sa.ForeignKeyConstraint(["sender_id"], ["users.id"], name=op.f("fk_message_idempotency_keys_sender_id_users"), ondelete="CASCADE"),
+        sa.ForeignKeyConstraint(
+            ["message_id"],
+            ["messages.id"],
+            name=op.f("fk_message_idempotency_keys_message_id_messages"),
+            ondelete="CASCADE",
+        ),
+        sa.PrimaryKeyConstraint("id", name=op.f("pk_message_idempotency_keys")),
+        sa.UniqueConstraint("chat_id", "sender_id", "client_message_id", name="uq_msg_idem_chat_sender_client"),
+    )
+    op.create_index(op.f("ix_message_idempotency_keys_id"), "message_idempotency_keys", ["id"], unique=False)
+    op.create_index(op.f("ix_message_idempotency_keys_chat_id"), "message_idempotency_keys", ["chat_id"], unique=False)
+    op.create_index(
+        op.f("ix_message_idempotency_keys_sender_id"),
+        "message_idempotency_keys",
+        ["sender_id"],
+        unique=False,
+    )
+    op.create_index(
+        op.f("ix_message_idempotency_keys_message_id"),
+        "message_idempotency_keys",
+        ["message_id"],
+        unique=False,
+    )
+
+    op.create_table(
+        "message_receipts",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("chat_id", sa.Integer(), nullable=False),
+        sa.Column("user_id", sa.Integer(), nullable=False),
+        sa.Column("last_delivered_message_id", sa.Integer(), nullable=True),
+        sa.Column("last_read_message_id", sa.Integer(), nullable=True),
+        sa.Column("updated_at", sa.DateTime(timezone=True), server_default=sa.func.now(), nullable=False),
+        sa.ForeignKeyConstraint(["chat_id"], ["chats.id"], name=op.f("fk_message_receipts_chat_id_chats"), ondelete="CASCADE"),
+        sa.ForeignKeyConstraint(["user_id"], ["users.id"], name=op.f("fk_message_receipts_user_id_users"), ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("id", name=op.f("pk_message_receipts")),
+        sa.UniqueConstraint("chat_id", "user_id", name="uq_msg_receipts_chat_user"),
+    )
+    op.create_index(op.f("ix_message_receipts_id"), "message_receipts", ["id"], unique=False)
+    op.create_index(op.f("ix_message_receipts_chat_id"), "message_receipts", ["chat_id"], unique=False)
+    op.create_index(op.f("ix_message_receipts_user_id"), "message_receipts", ["user_id"], unique=False)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_message_receipts_user_id"), table_name="message_receipts")
+    op.drop_index(op.f("ix_message_receipts_chat_id"), table_name="message_receipts")
+    op.drop_index(op.f("ix_message_receipts_id"), table_name="message_receipts")
+    op.drop_table("message_receipts")
+
+    op.drop_index(op.f("ix_message_idempotency_keys_message_id"), table_name="message_idempotency_keys")
+    op.drop_index(op.f("ix_message_idempotency_keys_sender_id"), table_name="message_idempotency_keys")
+    op.drop_index(op.f("ix_message_idempotency_keys_chat_id"), table_name="message_idempotency_keys")
+    op.drop_index(op.f("ix_message_idempotency_keys_id"), table_name="message_idempotency_keys")
+    op.drop_table("message_idempotency_keys")