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>

schema.sql

@@ -42,6 +42,19 @@ CREATE INDEX IF NOT EXISTS idx_posts_root_date
   ON posts(post_date, post_state)
   WHERE parent_id IS NULL;
 
+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)
+);
+
 CREATE TABLE IF NOT EXISTS post_votes (
   user_id TEXT NOT NULL REFERENCES users(id),
   post_id TEXT NOT NULL REFERENCES posts(id) ON DELETE CASCADE,