Error types are now collected into a single place. Similar errors are
merged.
Request invariant violations are now all under 400 with type
"invalid_request" if it's a client mistake; and if it's caused by a
server restrction, under 403 with type "disabled".
We do not need that many big numbers yet. This should simplify client
implementation without native u64 (eg. JavaScript).
Also put a hard limit on `unseen_cnt` query.
This decouples SQLs from handler logic, makes it easier for auditing and
caching. It also enables the possibility to switch or support multiple
database backends.
- `Msg` or `msg` is now the canonical term for the substructure in a
room. It includes a `chat` subtype and (in the future) other
administration subtypes like member joining or leaving.
- `Message` or `message` can used in human oriented context like docs
and comments, but only when it is unambiguous.
- `message` is not chosen in code because it's hard to type (at least
for me!), and have ambiguous meaning of:
- "Human readable text" in context of `ApiError`'s field.
- "A unit of data transfer, datagram" in context of WebSocket Message.
- `item` is not chosen because it is overly generic.
