costco-watch: client-side browser headers, drop Transform Rule requirement

apps/costco-watch/build/README.md

@@ -5,9 +5,12 @@ Source: https://git.alexzaw.dev/alexz/costco-watch (working copy:
 ~/projects/costco-watch). Build + push with ./build.sh.
 
 Gotchas:
-- TARGET_URL must go through the costco-tire cloudflared proxy AND that
-  hostname needs a Cloudflare Transform Rule adding browser headers
-  (User-Agent, Accept, Accept-Language, Sec-Fetch-*). Without it Costco stalls
-  the request until timeout and the checker reports "error"/"blocked".
+- Costco stalls (no response, just a hang) on requests lacking Sec-Fetch-*
+  headers. watch.js sends a browser header set in BROWSER_HEADERS - do not
+  strip it. Any single Sec-Fetch-* header is sufficient; UA/Accept alone are
+  not. Cloudflare Transform Rules CANNOT substitute for this: Sec-* headers are
+  on Cloudflare's restricted list, and cloudflared can't add headers at all.
+- TARGET_URL goes through the costco-tire cloudflared ingress, which sets
+  httpHostHeader: tires.costco.com.
 - State (last stock value, transition history) lives in ${APP_DATA_DIR}/data;
   wiping it means the next check can re-alert.

apps/costco-watch/config.json

@@ -13,7 +13,7 @@
     "automation",
     "utilities"
   ],
-  "description": "Polls a Costco tire product page on an interval and pushes an ntfy alert the moment it flips from out-of-stock to in-stock. Points at the costco-tire cloudflared proxy, which needs a Cloudflare Transform Rule attaching browser-like request headers - Costco stalls non-browser header sets.",
+  "description": "Polls a Costco tire product page on an interval and pushes an ntfy alert the moment it flips from out-of-stock to in-stock. Sends browser fetch-metadata headers itself - Costco stalls requests without Sec-Fetch-*, and neither cloudflared nor Cloudflare Transform Rules can supply them (Sec-* is on Cloudflare's restricted list). No Cloudflare-side configuration needed.",
   "short_desc": "Alerts when a Costco tire is back in stock",
   "author": "alexz",
   "source": "https://git.alexzaw.dev/alexz/costco-watch",

apps/costco-watch/metadata/description.md

@@ -3,19 +3,28 @@
 Polls a Costco tire product page on an interval and pushes an ntfy alert when
 the item flips from out-of-stock to in-stock.
 
-## Why it points at a proxy, not costco.com
+## The header gotcha
 
-Costco's bot mitigation silently stalls requests carrying a non-browser header
-set (the connection completes TLS, then nothing comes back - no 403, just a
-hang). The page loads fine through `costco-tire.alexzaw.dev`, a cloudflared
-ingress rule with `httpHostHeader: tires.costco.com`, **provided** a Cloudflare
-Transform Rule attaches browser-like request headers (`User-Agent`, `Accept`,
-`Accept-Language`, `Sec-Fetch-*`, `Upgrade-Insecure-Requests`) for that
-hostname. cloudflared itself cannot add headers - its `originRequest` block has
-no such option - which is why the rule lives at the edge.
+Costco's bot mitigation silently stalls requests that lack browser
+fetch-metadata headers - TLS completes, then nothing comes back, no 403, just a
+hang until timeout. Bisecting against the live site showed the gate is the
+`Sec-Fetch-*` family: **any single one** of `Sec-Fetch-Site`, `Sec-Fetch-Mode`
+or `Sec-Fetch-Dest` is enough to get a normal response, even with no
+`User-Agent` at all. User-Agent, Accept and Accept-Language on their own make
+no difference.
 
-If the checker starts reporting `blocked`, that Transform Rule is the first
-thing to verify.
+This service therefore sends a browser header set itself (see
+`BROWSER_HEADERS` in `watch.js`). Two things that do **not** work as fixes:
+
+- cloudflared cannot add headers - its `originRequest` block has no such
+  option, it only forwards what the client sent.
+- Cloudflare Transform Rules cannot set `Sec-*` headers - they're on
+  Cloudflare's restricted list, so the edge can't supply the one header family
+  that matters.
+
+`TARGET_URL` still points at the `costco-tire.alexzaw.dev` cloudflared ingress
+(which sets `httpHostHeader: tires.costco.com`), but no Cloudflare-side
+configuration is required.
 
 ## Config