fix(config-sync): alt.value rejected as string after website#62 migration

[d4rken]

After airplanes-live/website#62 migrated config_state['alt']['value'] to float metres AMSL end-to-end, the remote-config-sync POST from the feeder is rejected by the server:

apl-feed-config-sync level=error reason=validation_failed
  body={"error":"validation_failed","errors":{"fields":{"alt":"alt.value must be a number or null"}}}

feed/scripts/apl-feed/config.sh builds the outbound payload with jq --arg alt_v "$alt", which always emits a JSON string ("120m", "42.5", etc.). The website serializer in accounts/serializers/feeder.py requires int | float | None for alt.value. Every sync tick fails until REMOTE_CONFIG_ENABLED is turned off again.

The website docstring claimed feed + Go webconfig were "migrated in lockstep" — the lockstep migration on the feed side never landed.

Scope

The fix is a contract alignment: store altitude as bare metres on disk too, not just on the wire. Today's three-layer setup (unit-suffixed disk, suffix-tolerant operator input, metres-on-wire) creates an asymmetric round-trip — an operator who types 400ft ends up with 121.92m on disk after the first successful sync, with no notification.

Storage flips to bare metres end-to-end via a single shared altitude_to_bare_metres helper in scripts/lib/configure-validators.sh, used by apply, configure.sh, the new on-update migrator, and the config-sync outbound payload. Operator input stays suffix-tolerant (120m / 400ft / 120 all accepted). Range-gate moves to post-conversion metres [-1000, 10000] to match the website's contract exactly.

airplanes-live/image-webconfig follows in a second PR with the matching Go canonicalizer, SPA dirty-state comparator, and a dynamic m unit indicator next to the altitude input.

[d4rken]

Fixed on dev by #103 (store ALTITUDE as bare metres end-to-end) and #104 (drop the on-disk migrator in favour of defensive conversion on the wire).

The outbound config-sync payload now emits alt.value as a JSON number — config.sh runs the value through a shared altitude_to_bare_metres helper (scripts/lib/configure-validators.sh) and applies | tonumber in the jq filter. Disk storage is canonicalised to bare metres via the apply layer, operator input stays suffix-tolerant (120m / 400ft / 120), and legacy suffixed on-disk values are converted defensively on send. Covered by test_apl_feed_config_sync.bats and test_altitude_to_bare_metres.bats.