[feat] SQLite Support

[ktwrd]

This PR adds support for SQLite. Existing configuration files do not need to be updated, but they should be to prevent a broken configuration file in the future.
To migrate to the new config file format for the Database element, you can do the following:

diff --git a/config.xml b/config.xml
index 4850922..6de7ade 100644
--- a/config.xml
+++ b/config.xml
@@ -1,8 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <Kasta>
-    <Database Host="localhost" Port="5432" Name="kasta-dev">
-        <Username>kasta-dev</Username>
-        <Password>kasta</Password>
+    <Database Provider="Postgres">
+        <Postgres Host="localhost" Port="5432" Name="kasta-dev">
+            <Username>kasta-dev</Username>
+            <Password>kasta</Password>
+        </Postgres>
     </Database>

This PR also migrates Kasta from using DateTimeOffset to DateTime (UTC only) in the DB to support SQLite.

By default, Kasta will use SQLite for its database, but it is strongly recommended to use Postgres for non-development or testing purposes.

For docker installations, the database filename will default to /config/kasta.db, but for non-docker installations it'll default to kasta.db in the current working directory.

---

Resolves #13