Limit JSON payload and register token size

[juanfradb]

Summary

• add an explicit 8 KiB JsonConfig default for API JSON extractors
• cap /api/register token values at 4096 bytes
• keep /api/register body bounded at 128 KiB so oversized-token validation can return 400 while 1 MB payloads return 413
• document request body and token limits in docs/api.md

Security validation

Source/sink/control: attacker-controlled JSON bodies and register token values reached Actix JSON extraction and token storage without explicit size controls. The app now configures bounded JSON extraction and rejects register tokens above 4096 bytes before storage.

Original issue no longer reproduces:

• 1 MB JSON payload to /api/register returns 413
• 100 KiB token in /api/register returns 400
• non-register JSON body above the 8 KiB default returns 413

Refs #8

Tests

• cargo test payload -- --nocapture
• cargo test register_hundred_kb_token_returns_400 -- --nocapture
• cargo test
• cargo fmt --check
• cargo clippy --all-targets -- -D warnings
• git diff --check

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Reviewing files that changed from the base of the PR and between 893e7e1 and 5293500.

📒 Files selected for processing (4)

• docs/api.md
• src/api/routes.rs
• src/api/test_support.rs
• src/main.rs

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[AndreaDiazCorreia]

@codex review

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Codex Review: Didn't find any major issues. Nice work!

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[AndreaDiazCorreia]

Good job but Before merging: the 413 body comes back as text/plain and echoes the exact server limit (JSON payload (X bytes) is larger than allowed (limit: Y bytes)), which both breaks the JSON everywhere shape and gives free fingerprinting, so please add an error_handler on both JsonConfigs returning something like {"success":false,"message":"Payload too large"}. Also missing a test for the 413 path on /api/notify covering registered vs unregistered plus x-request-id presence, which matters given that endpoint's privacy contract. Optional nits: 128 KiB on /api/register is generous (16 to 32 KiB would do the same job with less parse surface), and JSON_PAYLOAD_LIMIT_BYTES could be renamed DEFAULT_JSON_PAYLOAD_LIMIT_BYTES for clarity.