Add unified post storage (#3)

Adds one post creation path for roots and replies, recursive thread loading, body-only updates, root listings, and root-only voting across PostgreSQL and the in-memory store.

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

internal/store/sqlc/models.go

@@ -25,7 +25,7 @@ type Post struct {
 	Body      string
 	City      string
 	PostDate  string
-	Hidden    int32
+	PostState string
 	CreatedAt string
 	UpdatedAt string
 }