refactor(database)!: decouple SQLs from backend logic and cache stmts

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.

blah-types/src/lib.rs

@@ -490,6 +490,7 @@ bitflags::bitflags! {
 
     #[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
     pub struct RoomAttrs: u64 {
+        // NB. Used by schema.
         const PUBLIC_READABLE = 1 << 0;
         const PUBLIC_JOINABLE = 1 << 1;