feat(backend): chat list marks a deleted last message, history pages mark messages with a note

backend/migrations/versions/f1c4a8e2d6b3_chat_stats_last_deleted.py

@@ -0,0 +1,97 @@
+"""chat stats last deleted
+
+Revision ID: f1c4a8e2d6b3
+Revises: e8b3f1a6c2d4
+Create Date: 2026-10-19 04:00:00.000000
+
+"""
+
+from collections.abc import Sequence
+
+from alembic import op
+
+revision: str = "f1c4a8e2d6b3"
+down_revision: str | None = "e8b3f1a6c2d4"
+branch_labels: str | Sequence[str] | None = None
+depends_on: str | Sequence[str] | None = None
+
+_APPLY = """
+CREATE OR REPLACE FUNCTION chat_stats_apply() RETURNS trigger LANGUAGE plpgsql AS $$
+BEGIN
+    IF TG_OP = 'INSERT' THEN
+        INSERT INTO chat_stats AS cs (account_id, chat_id, message_count,
+                                      last_date, last_message_id,
+                                      last_text, last_sender_id{column})
+        VALUES (NEW.account_id, NEW.chat_id, 1,
+                CASE WHEN NEW.date <= now() + interval '1 day'
+                     THEN NEW.date END,
+                CASE WHEN NEW.date <= now() + interval '1 day'
+                     THEN NEW.message_id END,
+                NEW.text, NEW.sender_id{value})
+        ON CONFLICT (account_id, chat_id) DO UPDATE SET
+            message_count = cs.message_count + 1,
+            last_date = CASE WHEN chat_stats_newer(cs, EXCLUDED)
+                        THEN EXCLUDED.last_date ELSE cs.last_date END,
+            last_message_id = CASE WHEN chat_stats_newer(cs, EXCLUDED)
+                              THEN EXCLUDED.last_message_id
+                              ELSE cs.last_message_id END,
+            last_text = CASE WHEN chat_stats_newer(cs, EXCLUDED)
+                        THEN EXCLUDED.last_text ELSE cs.last_text END,
+            last_sender_id = CASE WHEN chat_stats_newer(cs, EXCLUDED)
+                             THEN EXCLUDED.last_sender_id
+                             ELSE cs.last_sender_id END{merge};
+    ELSE
+        UPDATE chat_stats
+        SET last_text = NEW.text, last_sender_id = NEW.sender_id{update}
+        WHERE account_id = NEW.account_id
+          AND chat_id = NEW.chat_id
+          AND last_message_id = NEW.message_id;
+    END IF;
+    RETURN NULL;
+END;
+$$
+"""
+
+_DELETED_PARTS = {
+    "column": ", last_deleted",
+    "value": ", NEW.deleted_at IS NOT NULL",
+    "merge": (
+        ",\n            last_deleted = CASE WHEN chat_stats_newer(cs, EXCLUDED)"
+        " THEN EXCLUDED.last_deleted ELSE cs.last_deleted END"
+    ),
+    "update": ", last_deleted = NEW.deleted_at IS NOT NULL",
+    "deleted": " OR OLD.deleted_at IS DISTINCT FROM NEW.deleted_at",
+}
+_PLAIN_PARTS = dict.fromkeys(_DELETED_PARTS, "")
+
+_UPDATE_TRIGGER = (
+    "CREATE TRIGGER messages_chat_stats_update AFTER UPDATE ON messages "
+    "FOR EACH ROW WHEN (OLD.text IS DISTINCT FROM NEW.text "
+    "OR OLD.sender_id IS DISTINCT FROM NEW.sender_id{deleted}) "
+    "EXECUTE FUNCTION chat_stats_apply()"
+)
+
+_BACKFILL = """
+UPDATE chat_stats cs SET last_deleted = true
+FROM messages m
+WHERE m.account_id = cs.account_id AND m.chat_id = cs.chat_id
+  AND m.message_id = cs.last_message_id AND m.date = cs.last_date
+  AND m.deleted_at IS NOT NULL
+"""
+
+
+def upgrade() -> None:
+    op.execute(
+        "ALTER TABLE chat_stats ADD COLUMN last_deleted boolean NOT NULL DEFAULT false"
+    )
+    op.execute(_BACKFILL)
+    op.execute(_APPLY.format(**_DELETED_PARTS))
+    op.execute("DROP TRIGGER IF EXISTS messages_chat_stats_update ON messages")
+    op.execute(_UPDATE_TRIGGER.format(**_DELETED_PARTS))
+
+
+def downgrade() -> None:
+    op.execute("DROP TRIGGER IF EXISTS messages_chat_stats_update ON messages")
+    op.execute(_UPDATE_TRIGGER.format(**_PLAIN_PARTS))
+    op.execute(_APPLY.format(**_PLAIN_PARTS))
+    op.execute("ALTER TABLE chat_stats DROP COLUMN last_deleted")