feat(contacts): add contacts module with backend APIs and web tab

- add user_contacts table and migration

- expose /users/contacts list/add/remove endpoints

- add Contacts tab in chat list with add/remove actions

alembic/versions/0017_user_contacts.py

@@ -0,0 +1,42 @@
+"""add user contacts table
+
+Revision ID: 0017_user_contacts
+Revises: 0016_chat_invites
+Create Date: 2026-03-08 23:10:00.000000
+"""
+
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = "0017_user_contacts"
+down_revision: Union[str, Sequence[str], None] = "0016_chat_invites"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "user_contacts",
+        sa.Column("id", sa.Integer(), nullable=False),
+        sa.Column("user_id", sa.Integer(), nullable=False),
+        sa.Column("contact_user_id", sa.Integer(), nullable=False),
+        sa.Column("created_at", sa.DateTime(timezone=True), server_default=sa.text("now()"), nullable=False),
+        sa.ForeignKeyConstraint(["user_id"], ["users.id"], name=op.f("fk_user_contacts_user_id_users"), ondelete="CASCADE"),
+        sa.ForeignKeyConstraint(["contact_user_id"], ["users.id"], name=op.f("fk_user_contacts_contact_user_id_users"), ondelete="CASCADE"),
+        sa.PrimaryKeyConstraint("id", name=op.f("pk_user_contacts")),
+        sa.UniqueConstraint("user_id", "contact_user_id", name="uq_user_contacts_pair"),
+    )
+    op.create_index(op.f("ix_user_contacts_id"), "user_contacts", ["id"], unique=False)
+    op.create_index(op.f("ix_user_contacts_user_id"), "user_contacts", ["user_id"], unique=False)
+    op.create_index(op.f("ix_user_contacts_contact_user_id"), "user_contacts", ["contact_user_id"], unique=False)
+
+
+def downgrade() -> None:
+    op.drop_index(op.f("ix_user_contacts_contact_user_id"), table_name="user_contacts")
+    op.drop_index(op.f("ix_user_contacts_user_id"), table_name="user_contacts")
+    op.drop_index(op.f("ix_user_contacts_id"), table_name="user_contacts")
+    op.drop_table("user_contacts")
+