feat(02-03): Worker entry — webhook route + /api/session security gate + deploy plumbing (D-11, D-14, D-16, D-17)

- src/webhook.ts: routes exactly /telegram/webhook (grammY cloudflare-mod
  adapter, constant-time secret-token auth against env.WEBHOOK_SECRET) and
  POST /api/session (validateInitData → 401 category-only rejections →
  mintSession one-time token; single mint path, no extra branch — T-02-01/02/06)
- D-14: the only log statement is '[initdata] rejected: <category>' — no
  payload, ids, or timestamps
- scripts/set-webhook.mjs: setWebhook with secret_token + allowed_updates
  [message], secrets read from gitignored .dev.vars, token never echoed
- bot deploy script chains wrangler deploy + set-webhook; wrangler.jsonc
  gains workers_dev (webhook URL target); KV id stays placeholder for the
  deploy checkpoint
- verified: 27/27 tests, typecheck clean, wrangler deploy --dry-run exit 0

bot/src/webhook.ts

@@ -0,0 +1,65 @@
+/*
+ * webhook.ts — the Worker entry (D-11, D-14, D-16, D-17).
+ * --------------------------------------------------------
+ * Routes exactly two paths:
+ *   POST /telegram/webhook  → grammY's webhook adapter (cloudflare-mod) with
+ *     the configured secret token — Telegram's updates must carry
+ *     X-Telegram-Bot-Api-Secret-Token = env.WEBHOOK_SECRET or they are 401'd
+ *     by a constant-time compare BEFORE any bot processing or KV touch
+ *     (T-02-03). The bot is instantiated per request because the token only
+ *     arrives with `env` (research-verified Workers pattern).
+ *   POST /api/session       → the phase's security gate (T-02-01): the caller
+ *     posts a verbatim Telegram.WebApp.initData string; only an HMAC-validated,
+ *     fresh identity obtains a one-time opaque session token (D-06). Rotation
+ *     of any prior token for the chat is mintSession's own invariant (D-07) —
+ *     this endpoint adds no extra branch and no extra write path (T-02-06).
+ * Anything else → 404.
+ *
+ * D-14: the ONLY log statement is category-only (malformed/tampered/stale) —
+ * never the raw payload, never user ids, never timestamps. Rejection
+ * responses carry the category only.
+ * D-17: no secret is ever logged or echoed; the bot token / webhook secret
+ * exist only inside `env` at runtime.
+ */
+import { webhookCallback } from "grammy";
+import type { Env } from "./config.ts";
+import { createBot } from "./bot/bot.ts";
+import { mintSession } from "./kv/sessions.ts";
+import { validateInitData } from "./security/initdata.ts";
+
+export default {
+  async fetch(req: Request, env: Env, _ctx: ExecutionContext): Promise<Response> {
+    const url = new URL(req.url);
+    if (url.pathname === "/telegram/webhook") {
+      const bot = createBot(env); // env-bound token → per-request instantiation
+      const cb = webhookCallback(bot, "cloudflare-mod", undefined, undefined, env.WEBHOOK_SECRET);
+      return cb(req);
+    }
+    if (url.pathname === "/api/session") {
+      return handleSession(req, env);
+    }
+    return new Response("Not found", { status: 404 });
+  },
+} satisfies ExportedHandler<Env>;
+
+/** Phase-2 security gate: initData in, one-time opaque session token out —
+ *  nothing else; unvalidated identities get a category-only 401 (T-02-01). */
+async function handleSession(req: Request, env: Env): Promise<Response> {
+  if (req.method !== "POST") return new Response("Method not allowed", { status: 405 });
+  const raw = await req.text(); // the verbatim Telegram.WebApp.initData string
+  const result = await validateInitData(raw, env.BOT_TOKEN);
+  if (!result.ok) {
+    console.log(`[initdata] rejected: ${result.reason}`); // D-14: category ONLY — no payload, no ids, no timestamps
+    return new Response(JSON.stringify({ error: result.reason }), {
+      status: 401,
+      headers: { "content-type": "application/json" },
+    });
+  }
+  const token = await mintSession(env.chat_sessions, {
+    user_id: result.user.id,
+    chat_id: result.chat?.id ?? result.user.id, // private chat: Telegram conflates; claim surface unchanged (D-08)
+    issued_at: Math.floor(Date.now() / 1000),
+    language_code: result.languageCode,
+  });
+  return new Response(JSON.stringify({ token }), { status: 200, headers: { "content-type": "application/json" } });
+}