feat: scaffold Flask + React app with schema and JSON error handling

Claude-Session: https://claude.ai/code/session_01M9MLit5Ko3X4s7rzC5Kv7X

backend/schema.sql

@@ -0,0 +1,52 @@
+CREATE TABLE IF NOT EXISTS users (
+    id            SERIAL PRIMARY KEY,
+    username      TEXT NOT NULL,
+    password_hash TEXT NOT NULL,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+CREATE UNIQUE INDEX IF NOT EXISTS users_username_lower_key ON users (lower(username));
+
+CREATE TABLE IF NOT EXISTS sessions (
+    token_hash BYTEA PRIMARY KEY,
+    user_id    INT NOT NULL REFERENCES users ON DELETE CASCADE,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    expires_at TIMESTAMPTZ NOT NULL
+);
+CREATE INDEX IF NOT EXISTS sessions_user_id_idx ON sessions (user_id);
+
+CREATE TABLE IF NOT EXISTS genres (
+    id   SMALLSERIAL PRIMARY KEY,
+    name TEXT NOT NULL UNIQUE
+);
+
+-- WHERE NOT EXISTS instead of ON CONFLICT: ON CONFLICT consumes a sequence value per row
+-- on every startup, which would eventually overflow SMALLSERIAL.
+INSERT INTO genres (name)
+SELECT seed.name
+  FROM (VALUES ('Fiction'), ('Non-Fiction'), ('Mystery'), ('Thriller'), ('Science Fiction'),
+               ('Fantasy'), ('Romance'), ('Horror'), ('Historical Fiction'), ('Biography'),
+               ('Memoir'), ('Self-Help'), ('Science'), ('History'), ('Poetry'), ('Other')) AS seed(name)
+ WHERE NOT EXISTS (SELECT 1 FROM genres WHERE genres.name = seed.name);
+
+CREATE TABLE IF NOT EXISTS books (
+    id           SERIAL PRIMARY KEY,
+    user_id      INT NOT NULL REFERENCES users ON DELETE CASCADE,
+    title        TEXT NOT NULL CHECK (btrim(title) <> ''),
+    author       TEXT NOT NULL CHECK (btrim(author) <> ''),
+    genre_id     SMALLINT NOT NULL REFERENCES genres,
+    total_pages  INT NOT NULL CHECK (total_pages > 0),
+    current_page INT NOT NULL DEFAULT 0,
+    created_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at   TIMESTAMPTZ NOT NULL DEFAULT now(),
+    CONSTRAINT books_current_page_range CHECK (current_page BETWEEN 0 AND total_pages)
+);
+CREATE INDEX IF NOT EXISTS books_user_id_idx ON books (user_id);
+
+CREATE TABLE IF NOT EXISTS notes (
+    id         SERIAL PRIMARY KEY,
+    book_id    INT NOT NULL REFERENCES books ON DELETE CASCADE,
+    body       TEXT NOT NULL CHECK (btrim(body) <> ''),
+    created_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+CREATE INDEX IF NOT EXISTS notes_book_id_idx ON notes (book_id);