feat(messages): end-to-end encrypted real-time direct messaging backend

Private user-to-user messages are end-to-end encrypted: the server only ever
stores ciphertext + iv and acts as a key directory (public keys) plus an opaque
store for each user's private key, wrapped client-side with a passphrase the
server never sees — so not even an admin can read a conversation. A separate
kind=system message (plaintext, no sender) powers a server-authored Siftlode
welcome shown on first open, reusable later for announcements.

- models: rework Message (kind, nullable sender/body, ciphertext+iv) + MessageKey;
  migrations 0026 (table) + 0027 (E2EE rework).
- routes/messages.py: key directory/blob endpoints, ciphertext send, conversations
  + threads (system + user), lazy welcome, all gated by require_human.
- realtime.py: in-process WebSocket connection registry; /ws delivers sent
  messages to a user's open tabs instantly (sync-callable push, single-process).

backend/alembic/versions/0026_messages.py

@@ -0,0 +1,56 @@
+"""user-to-user messages
+
+Revision ID: 0026_messages
+Revises: 0025_install_wizard
+Create Date: 2026-06-25
+
+Adds the `messages` table for direct user-to-user messaging (notification module phase 2).
+A conversation is the set of rows between two users; both FKs cascade so a GDPR account
+erasure removes that user's messages.
+"""
+from typing import Sequence, Union
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0026_messages"
+down_revision: Union[str, None] = "0025_install_wizard"
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "messages",
+        sa.Column("id", sa.Integer(), primary_key=True),
+        sa.Column(
+            "sender_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column(
+            "recipient_id",
+            sa.Integer(),
+            sa.ForeignKey("users.id", ondelete="CASCADE"),
+            nullable=False,
+        ),
+        sa.Column("body", sa.Text(), nullable=False),
+        sa.Column("read_at", sa.DateTime(timezone=True), nullable=True),
+        sa.Column(
+            "created_at",
+            sa.DateTime(timezone=True),
+            server_default=sa.func.now(),
+            nullable=False,
+        ),
+    )
+    op.create_index("ix_messages_sender_id", "messages", ["sender_id"])
+    op.create_index("ix_messages_recipient_id", "messages", ["recipient_id"])
+    op.create_index("ix_messages_created_at", "messages", ["created_at"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_messages_created_at", table_name="messages")
+    op.drop_index("ix_messages_recipient_id", table_name="messages")
+    op.drop_index("ix_messages_sender_id", table_name="messages")
+    op.drop_table("messages")