docs: a 0.3 → 0.4 migration guide, and make the configuration example demonstrate configuration

[Kilerd]

Three gaps found while reviewing whether 0.4 was ready to publish.

1. MIGRATION.md was stale — and had become wrong

It documented only 0.2 → 0.3, and line 177 still showed:

DatabasePool::connect(&config.application.database_url).await

which no longer compiles in 0.4. Meanwhile 0.4 carries the most invasive breaking change the project has had — every application must edit its configuration file — and the repository offered no guide for it at all. The release notes covered it; the file a user actually opens when upgrading did not.

Adds a 0.3 → 0.4 section:

• configuration file layout ([basic]/[application] → top level + [server]), with a before/after
• reading config in code (config.application.name → config.name), and #[config] / State<ServerConfig>
• the __ environment separator, and that typed fields now parse instead of failing the load
• the removed message feature, and that cors / static_files now enable only their own half of tower-http
• smaller changes: 422 validation rejections, E: ErrorResponsible, unit-returning handlers, Operable fields

The 0.2 → 0.3 section is kept and labelled — its examples are correct for the version it describes, and it now says so.

2. The configuration example did not demonstrate configuration

.with_optional_config("config.toml")   // does not exist — and optional, so it failed silently
.listen("127.0.0.1:3000")              // hardcoded, ignoring the `[server] port = 8000` it ships

The one example named after the feature was the one not showing it. It now uses the trait API, extracts the application config with #[config] and the bind settings with State<ServerConfig>, and lets run() bind what the file says.

Verified by actually running it rather than only compiling:

$ curl :8000/          → Gotcha                         # top-level `welcome`
$ curl :8000/server    → listening on 127.0.0.1:8000    # the [server] section

$ APP_SERVER__PORT=9100 APP_WELCOME=from-env cargo run
$ curl :9100/          → from-env                       # single underscore = field name
$ curl :9100/server    → listening on 127.0.0.1:9100    # `__` = nested section

That also confirms end-to-end that the documented environment-override rules match real behaviour.

3. A todo!() in a production path

gotcha/src/router.rs matched MethodFilter with _ => todo!(). MethodFilter is #[non_exhaustive], so a method axum adds later would panic the application while it registered its routes. It now leaves such a route working and merely undocumented, with a warning — consistent with the panic removal in #40 and #31.

Verification

Workspace builds with --all-features, all feature combinations test, clippy --all-features --workspace, fmt, and the configuration example exercised over HTTP.

🤖 Generated with Claude Code