chore(firmware): mark TODOs for the dev-only 60s polling cap

Three cross-referenced markers — config.h, operation.h, and FW-10 in the
test file — calling out that the FETCH_INTERVAL_MS cap is intentionally
holding the polling rate at 1 minute for dev iteration. Once the firmware
is stable and we want the device to honor the app's per-frame
rotationIntervalMinutes / wakeHour settings, the cap in operation.h
becomes a sanity-clamp (e.g., 30 s ≤ sleep ≤ 25 h) and the no-header
fallback splits into its own constant.

Behavior unchanged — comments only.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/config.h

@@ -123,6 +123,18 @@
 #define AP_IP             "192.168.4.1"
 #define WIFI_TIMEOUT_MS   30000
 #ifndef FETCH_INTERVAL_MS
-#define FETCH_INTERVAL_MS 60000   // 1 min deep sleep between polls
+// TODO(post-dev): drop the 60s cap. Today this value is used in
+// operation.h as BOTH the no-header fallback AND the upper bound that
+// clamps the server-provided X-Interval-Ms. While we're iterating on the
+// firmware we want the frame to poll every minute so changes land fast,
+// but in production we want to honor whatever the app sends (e.g.,
+// rotationIntervalMinutes=60 → 1 hour, or wakeHour set → ~24 h sleep).
+// When the firmware stabilizes, split this into two constants:
+//   - FETCH_INTERVAL_MS_FALLBACK (used when no X-Interval-Ms header)
+//   - SLEEP_CLAMP_MIN_MS / SLEEP_CLAMP_MAX_MS (sanity bounds, not the
+//     primary schedule)
+// and let server values flow through. See operation.h:138 for the cap
+// site, and tests FW-09/FW-10 for the assertions that will need updating.
+#define FETCH_INTERVAL_MS 60000   // 1 min deep sleep between polls (DEV value)
 #endif
 #define IMAGE_PATH        "/img.bin"