feat(backend): pg schema + store queries incl. path-keyed progress

backend/internal/db/schema.sql

@@ -0,0 +1,21 @@
+CREATE TABLE IF NOT EXISTS users (
+  id BIGSERIAL PRIMARY KEY, username TEXT UNIQUE NOT NULL,
+  password_hash TEXT NOT NULL, role TEXT NOT NULL CHECK (role IN ('admin','member')),
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now());
+CREATE TABLE IF NOT EXISTS libraries (
+  id BIGSERIAL PRIMARY KEY, name TEXT NOT NULL, root_path TEXT UNIQUE NOT NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now());
+CREATE TABLE IF NOT EXISTS books (
+  id BIGSERIAL PRIMARY KEY, library_id BIGINT NOT NULL REFERENCES libraries(id),
+  path TEXT NOT NULL, title TEXT NOT NULL,
+  format TEXT NOT NULL CHECK (format IN ('cbz','pdf','epub','txt','md')),
+  file_size BIGINT NOT NULL, mod_ts BIGINT NOT NULL, page_count INT NOT NULL DEFAULT 0,
+  state TEXT NOT NULL DEFAULT 'ready' CHECK (state IN ('ready','error')),
+  error_msg TEXT NOT NULL DEFAULT '', added_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  UNIQUE (library_id, path));
+CREATE TABLE IF NOT EXISTS reading_progress (
+  user_id BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  library_id BIGINT NOT NULL, book_path TEXT NOT NULL,
+  locator JSONB NOT NULL DEFAULT '{}', percent DOUBLE PRECISION NOT NULL DEFAULT 0,
+  updated_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+  PRIMARY KEY (user_id, library_id, book_path));