core: scheduled deletion (#1075)

Co-authored-by: Evgeny Poberezkin <2769109+epoberezkin@users.noreply.github.com>

docs/rfcs/2022-09-20-chat-history-deletion.md

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+# Chat history deletion
+
+## Problem
+
+Currently chat history is preserved indefinitely and can only be cleared manually, either individual messages or per chat. This leads to privacy concerns as well as growing memory consumption.
+
+## Solution
+
+An option to turn on scheduled deletion of chat history (chat items and files). Initially only as a global setting but we can also consider setting per conversation.
+
+## Implementation plan
+
+Scheduled deletion implementation plan:
+
+- Enum ChatItemTTL - None, Day, Week, Month, etc.
+
+- Functions to convert ChatItemTTL to number of seconds for chatItemTTL and expireChatItemsInterval
+
+- Interval can be:
+
+  - TTL / 2
+  - depend on TTL (e.g. 1 day for a 1 week TTL) (reuse ExpirationConfig and have fixed configs?)
+  - fixed - 30 min?
+
+- iOS is not a long running process so we have to check after start
+
+- To prevent NSE from running this process parameterize startChat to allow starting without scheduled deletion even if it is configured
+
+- Don't update chats and previews?
+
+### Chat global expiration
+
+Api:
+
+  - API Command - SetChatItemTTL ChatItemTTL, response is CRCmdOk
+  - API Command - GetChatItemTTL, response is CRChatItemTTL
+
+UI:
+
+  - New view in settings, on start GetChatItemTTL to load into model
+  - When changed in UI - SetChatItemTTL, update in model
+  - UI options match ChatItemTTL
+
+Core:
+
+  - Add expireChatItems to ChatController: TVar (Maybe (Async ())) similar to agentAsync?
+  - Thread is created/stopped in runtime because interval has to be figured out dynamically when TTL is changed (e.g. if it was changed from 1 week to 30 mins and interval for 1 week is 1 day, we shouldn't wait 1 day before reading new interval)
+  - Add table settings, field chat_item_ttl
+  - On chat start - read settings, convert chat_item_ttl into chatItemTTL and expireChatItemsInterval (may be Nothing); if not Nothing - run expireMessages thread and put into controller
+  - On SetChatItemTTL - update settings
+    - If Nothing - cancel expireMessages, remove from controller, update setting in store
+    - If Just - start expireMessages, put into controller, update setting in store
+  - expireMessages thread:
+    forever $ do
+      threadDelay interval
+      expiration logic
+  - Expiration logic:
+    - Select all (chat ref, chat item id) older than (current time - TTL), comparing with updated_at (created_at?)
+    - Reuse logic from APIDeleteChatItem to delete each item (should messages be deleted or updated to XMsgDeleted?)
+
+#### Questions
+
+- single thread (don't re-create on change), read flag on each cycle and on each chat item
+- if ttl changed from none to some value - first run sync, no delay between chat items on first run
+- seconds instead of enum in api / backend
+- part of APISetChatSettings api? - unclear can block for long on first deletion
+- fixed interval
+- if ttl became smaller, set flag to false, then one sync cycle
+
+### Per chat expiration
+
+API:
+
+  - API Command - SetChatCITTL ChatRef ChatItemTTL, response is CRCmdOk
+  - API Command - GetChatCITTL ChatRef, response is CRChatItemTTL
+  - If we do both global and contact API can be SetChatItemTTL (Maybe ChatRef) ChatItemTTL or SetChatItemTTL GlobalOrChatRef ChatItemTTL, same for Get
+
+UI:
+
+  - In UI - in ChatInfo views, loaded on opening
+
+Core:
+
+  - Add expireChatCIs in ChatController: map [ChatRef, Async ()]
+  - Added and started/cancelled by chatRef
+  - Saved in contacts/groups tables
+  - On chat start - read from contacts/groups
+  - Expiration logic: select per chat