Rewrite backend in Go with JSON API and JS frontend

Reimplements the Flask app as a Go HTTP API (chi + modernc sqlite) with a
minimal vanilla-JS frontend in web/. Endpoints mirror the original Flask
routes but return JSON instead of HTML.

- auth: login/logout/register/sessioninfo/headerbar with crypto/rand tokens
- entry: paginated feed (single JOIN), create with image scaling
- user: profile, userinfo; delete still a stub
- requireAuth middleware passes uid via context
- notes/api.md documents the API and schema

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

db.go

@@ -0,0 +1,58 @@
+package main
+
+import (
+	"database/sql"
+
+	_ "modernc.org/sqlite"
+)
+
+var db *sql.DB
+
+const schema = `
+CREATE TABLE IF NOT EXISTS user (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    uid        INTEGER UNIQUE,
+    username   TEXT UNIQUE,
+    password   BLOB,
+    created_at INTEGER,
+    last_login INTEGER
+);
+
+CREATE TABLE IF NOT EXISTS session (
+    id          INTEGER PRIMARY KEY AUTOINCREMENT,
+    uid         INTEGER,
+    value       TEXT UNIQUE,
+    created_at  INTEGER,
+    expires     INTEGER,
+    description TEXT
+);
+
+CREATE TABLE IF NOT EXISTS entry (
+    id         INTEGER PRIMARY KEY AUTOINCREMENT,
+    pid        INTEGER UNIQUE,
+    uid        INTEGER,
+    created_at INTEGER,
+    content    TEXT,
+    filepath   TEXT
+);
+
+CREATE TABLE IF NOT EXISTS vote (
+    id   INTEGER PRIMARY KEY AUTOINCREMENT,
+    uid  INTEGER,
+    pid  INTEGER,
+    mode TEXT
+);
+`
+
+func initDB() error {
+	var err error
+	db, err = sql.Open("sqlite", "kver.db")
+	if err != nil {
+		return err
+	}
+	if err = db.Ping(); err != nil {
+		return err
+	}
+	_, err = db.Exec(schema)
+	return err
+}