Add unified posts database groundwork (#2)

Adds self-referencing post and post-vote tables, generated schema models, and an idempotent snapshot migration that preserves the legacy tables during the staged application cutover.

Co-authored-by: codegirl-007 <s.raide@gmail.com>

internal/store/sqlc/models.go

@@ -5,6 +5,7 @@
 package sqlc
 
 import (
+	"database/sql"
 	"time"
 )
 
@@ -16,6 +17,25 @@ type Answer struct {
 	UpdatedAt  string
 }
 
+type Post struct {
+	ID        string
+	ParentID  sql.NullString
+	AuthorID  string
+	Title     string
+	Body      string
+	City      string
+	PostDate  string
+	Hidden    int32
+	CreatedAt string
+	UpdatedAt string
+}
+
+type PostVote struct {
+	UserID string
+	PostID string
+	Value  int32
+}
+
 type Question struct {
 	ID        string
 	AuthorID  string