Support LUD-12 comments (commentAllowed)

[Chemaclass]

Context

The LNURL-pay params response hard-codes commentAllowed => false and the invoice callback ignores any comment query parameter.

• src/Invoice/Application/CallbackUrl.php — returns 'commentAllowed' => false with a // TODO: Not implemented yet.
• src/Invoice/Application/InvoiceGenerator.php — generateInvoice() builds the invoice; no comment handling.
• src/Invoice/Infrastructure/Controller/InvoiceController.php — reads only amount from the request.
• src/Invoice/InvoiceConfig.php / src/Config/LightningConfig.php — config reader/writer.

Goal

Implement LUD-12 comments: advertise a max comment length in the pay params, accept a comment on the callback, and attach it to the invoice.

Scope / Tasks

• Add a config setting for the max comment length (e.g. LightningConfig::setCommentAllowed(int $chars), default 0 = disabled) with a matching InvoiceConfig::getCommentAllowed(): int. Wire it through ConfigKey.
• CallbackUrl::getCallbackUrl() returns commentAllowed => <int> from config instead of the hard-coded false.
• InvoiceController reads the comment query param and passes it into generateInvoice().
• InvoiceGenerator validates the comment length against commentAllowed (reject over-length with the LNURL {status:ERROR, reason} shape) and uses the comment as/within the invoice memo.
• Update the facade/generator return-shape docblocks and the README HTTP API section.

Implementation notes

• Per LUD-12, commentAllowed is the number of characters accepted (0/absent = not allowed). Enforce the limit server-side.
• Follow the existing config pattern: private field + fluent setter + jsonSerialize() entry in LightningConfig, get(ConfigKey::…) in InvoiceConfig, both keyed via ConfigKey.
• Error responses already use {status:ERROR, reason:…} — reuse.

Acceptance criteria

• With commentAllowed unset, behavior is unchanged (commentAllowed => 0, comment ignored).
• With it set to N, pay params advertise commentAllowed => N; a ?comment= up to N chars is attached to the invoice; over-length is rejected with an LNURL error.
• Unit tests cover: advertised value, accepted comment, over-length rejection.
• composer test-all green.

References

• LUD-12: https://github.com/lnurl/luds/blob/luds/12.md
• LUD-06 error shape: { "status": "ERROR", "reason": "…" }