Switch from MySQL to SQLite for simpler deployment

Gemfile.lock

@@ -118,12 +118,11 @@ GEM
       net-smtp
     marcel (1.1.0)
     mini_mime (1.1.5)
+    mini_portile2 (2.8.9)
     minitest (6.0.1)
       prism (~> 1.5)
     msgpack (1.8.0)
     mutex_m (0.3.0)
-    mysql2 (0.5.7)
-      bigdecimal
     net-imap (0.6.2)
       date
       net-protocol
@@ -207,6 +206,8 @@ GEM
     reline (0.6.3)
       io-console (~> 0.5)
     securerandom (0.4.1)
+    sqlite3 (1.7.3)
+      mini_portile2 (~> 2.8.0)
     stringio (3.2.0)
     thor (1.5.0)
     timeout (0.6.0)
@@ -233,9 +234,12 @@ PLATFORMS
 DEPENDENCIES
   bootsnap
   debug
-  mysql2 (~> 0.5)
   puma (~> 6.0)
   rails (~> 7.1.0)
+  sqlite3 (~> 1.4)
+
+RUBY VERSION
+   ruby 3.4.7p58
 
 BUNDLED WITH
    2.6.9