db+adapter, add sqlx + schema & add adapter

Adds SQLX for database management, and an Adapter interface for
interacting with it. Through the type "AdapterPool"

Creates an initial_schema with everything I think I'll need.

flake.nix

@@ -2,7 +2,7 @@
   description = "Koucha rust flake";
 
   inputs = {
-    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11-small";
   };
 
   outputs = { self, nixpkgs }:
@@ -24,6 +24,8 @@
             rustc
             cargo
             rust-analyzer
+
+            sqlx-cli
           ];
 
           RUST_SRC_PATH = "${pkgs.rust.packages.stable.rustPlatform.rustLibSrc}";