move all mqtt publishing functions to mqtt module

- fix: use tagged enum serialization for TankError
- fix: rename TankInfo fields for consistent naming (volume_ml, pct, water_temp_c)
- renamed some fields for better clarity on contained value

rust/src/webserver/mod.rs

@@ -221,18 +221,12 @@ where
     let mut data_store = Vec::new();
     let mut total_read = 0;
     loop {
-        let left = max_read - total_read;
         let mut buf = [0_u8; 64];
-        let s_buf = if buf.len() <= left {
-            &mut buf
-        } else {
-            &mut buf[0..left]
-        };
-        let read = request.read(s_buf).await?;
+        let read = request.read(&mut buf).await?;
         if read == 0 {
             break;
         }
-        let actual_data = &s_buf[0..read];
+        let actual_data = &buf[0..read];
         total_read += read;
         if total_read > max_read {
             bail!("Request too large {total_read} > {max_read}");