Use a local SQLite3 backend instead of PostgreSQL Docker container.

* The provided SQLite3 database contains the required schemas, but no data and can be reset to if required.
* Remove all references to PostgreSQL in documentation and configuration.
* Replace native sqlite3 command with a console app to remove dependency on SQLite3 installation.

show-posts/Post.cs

@@ -0,0 +1,24 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+[Table("posts")]
+public class Post
+{
+    [Key]
+    [Column("id")]
+    public int Id { get; set; }
+
+    [Column("user_id")]
+    public int UserId { get; set; }
+
+    [Column("title")]
+    [Required]
+    public string Title { get; set; } = string.Empty;
+
+    [Column("body")]
+    [Required]
+    public string Body { get; set; } = string.Empty;
+
+    [Column("updated_at")]
+    public DateTimeOffset UpdatedAt { get; set; } = DateTimeOffset.Parse("1970-01-01T00:00:00Z"); // TODO: Remove the default value in Phase 3.
+}