Add post image storage (#10)

Adds migration 010, ordered attachment metadata, and transaction-safe image persistence for posts and replies.

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

internal/store/migrate.go

@@ -120,6 +120,25 @@ func migrateDropLegacyPostTables(ctx context.Context, exec execContext) error {
 	return nil
 }
 
+func migratePostImages(ctx context.Context, exec execContext) error {
+	if _, err := exec.ExecContext(ctx, `
+CREATE TABLE IF NOT EXISTS post_images (
+  id TEXT PRIMARY KEY,
+  post_id TEXT NOT NULL REFERENCES posts(id) ON DELETE CASCADE,
+  object_key TEXT NOT NULL UNIQUE,
+  public_url TEXT NOT NULL,
+  description TEXT NOT NULL DEFAULT '' CHECK (char_length(description) <= 500),
+  position SMALLINT NOT NULL CHECK (position BETWEEN 0 AND 3),
+  width INTEGER NOT NULL CHECK (width > 0),
+  height INTEGER NOT NULL CHECK (height > 0),
+  created_at TEXT NOT NULL,
+  UNIQUE (post_id, position)
+)`); err != nil {
+		return fmt.Errorf("create post images: %w", err)
+	}
+	return nil
+}
+
 func migratePostDate(ctx context.Context, exec execContext) error {
 	steps := []struct {
 		name string
@@ -311,6 +330,7 @@ CREATE TABLE IF NOT EXISTS schema_migrations (
 		{"007_post_state", migratePostState},
 		{"008_post_author_index", migratePostAuthorIndex},
 		{"009_drop_legacy_post_tables", migrateDropLegacyPostTables},
+		{"010_post_images", migratePostImages},
 	}
 	for _, m := range migrations {
 		if applied[m.version] {