feat: C++20 rewrite of hookla

[Yomanz]

Summary

• Full port of the Scala/Finch/Akka/Quill/Circe service to C++20, living in cpp/
• All three implemented providers carried over: GitHub (5 events), GitLab (6 events), Sonarr (4 events); Radarr and Ombi remain stubs matching the Scala original
• Same database schema — no migrations needed

Library mapping

Scala	C++
Finch / Finagle	Drogon v1.9 (C++20 co_await coroutines)
Akka actors	Removed — no actor-to-actor messaging in this service
Quill + async driver	libpqxx 7 with a mutex-guarded ConnectionPool
Circe	nlohmann/json (NLOHMANN_DEFINE_TYPE_NON_INTRUSIVE + manual enum overloads)
ackcord	cpp-httplib (SSLClient to discord.com:443)
MacWire	Explicit shared_ptr wiring in main.cpp
sbt	CMake 3.25 + FetchContent
distroless JVM (~180 MB)	debian:bookworm-slim (~30 MB, ~20 MB RSS idle)

Key structural changes from Scala

Handler dispatch — both provider-level and event-level dispatch use std::unordered_map<string, function<void(json, EventData)>> registered in each handler's constructor. No std::variant, no extra event class hierarchy. Adding a new event = one map entry.

BaseEvent class hierarchy collapsed — the Scala version had this layer because MacWire needed classes to wire. In C++ the event handlers are private methods on the provider handler (or free functions for complex ones like GitLab push).

JSON enums — each Scala StringEnum (GithubRefType, GithubCheckRunAction, SonarrEventType, etc.) becomes a C++ enum class + a hand-written from_json free function. ~100 lines that Circe/enumeratum generated automatically.

Nullable columns — each Option[T] in Scala models needs an explicit row["col"].is_null() check in EmbedOptionsService.cpp and friends.

Test plan

• cmake -B build && cmake --build build compiles cleanly with clang-17 or gcc-13
• docker build -f cpp/Dockerfile cpp/ produces a working image
• POST to /process/:token with a GitHub push payload returns 200 success and sends a Discord embed
• Invalid token returns 401
• Unknown event name logs a warning and returns 200 (graceful no-op)
• Sonarr test event (eventType: "Test" in body) sends the test embed