Add unified post storage.

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

internal/store/store.go

@@ -37,6 +37,13 @@ type Store interface {
 	GetAnswer(ctx context.Context, questionID string) (*Answer, error)
 	UpsertAnswer(ctx context.Context, a *Answer) error
 
+	CreatePost(ctx context.Context, post *Post) error
+	GetPost(ctx context.Context, id string) (*Post, error)
+	GetPostThread(ctx context.Context, rootID string) (*Post, error)
+	UpdatePost(ctx context.Context, post *Post) error
+	ListRootPosts(ctx context.Context, postDate, viewerID string) ([]Post, error)
+	VotePost(ctx context.Context, userID, postID string, value int) error
+
 	// Vote sets the vote to 1, -1, or 0 (clear) on a visible question.
 	Vote(ctx context.Context, userID, questionID string, value int) error
 }