Extract review logic into bot_actions module

Move `exec_review`, `download_git_diff`, and review formatting
to a new `bot_actions::review` module. Update the review flow to
post inline review comments via the Gitea API and simplify the
comment markdown to a summary. Add diff formatting that
preprocesses added lines with line numbers for the LLM prompt.

src/main.rs

@@ -2,6 +2,7 @@ use crate::{bot::Bot, gitea::WebhookType, state::AppState};
 
 mod api;
 mod bot;
+mod bot_actions;
 mod consts;
 mod env;
 mod errors;