Make HTTP body length tunable at runtime via APP_HTTP_MAX_BODY_LENGTH

[ngdangdat]

Summary

Follow-up to #265. The existing APP_HTTP_MAX_BODY_LENGTH env var is read via Application.compile_env, so it only takes effect at build time — flipping it on a running release has no impact. This PR makes it actually runtime-tunable, so operators can resize the limit by restarting the container with a different env value (no rebuild).

What changed

• New Lynx.Config.max_body_length/0 reads :http_max_body_length from app env with an 8 MB fallback.
• New LynxWeb.Plug.RuntimeParsers wraps Plug.Parsers and resolves length inside its own init/1 (so the value is fetched after runtime.exs runs).
• Endpoint now uses the wrapper instead of Plug.Parsers directly; the @max_body_length module attribute is gone.
• config :phoenix, :plug_init_mode, :runtime so endpoint plug init/1 callbacks run at endpoint boot rather than at module compile.
• config/runtime.exs reads APP_HTTP_MAX_BODY_LENGTH and writes it into the endpoint config.
• Default behavior unchanged: 8 MB when the env var is unset.

PoW

The state file is uploaded successfully. Dummy lock ID is used so Bad Request is expected.

• Environment variable is set to 50MiB
• Default value is 8MiB
• Dummy test file is 15MiB (should be failed if 8MiB is used for the configuration).

curl -v -X POST \
  -u "$USERNAME:$PASSWORD" \
  -H "Content-Type: application/json" \
  --data-binary @/tmp/15mb.bin \
  "https://${DOMAIN}/client/***/***/dev/state?ID=00000000-0000-0000-0000-000000000000"
....
* upload completely sent off: 15728640 bytes
...
<
Bad Request%

[ngdangdat]

@Clivern I'm very sorry for not testing the change carefully in the last PR 🙇. For this one, I built an image myself and tested if the change works. The result is put in PoW section of the PR.

[Clivern]

lgtm