Add the streamer-tools API client, with a real HTTP backend per platform

Implements the two read-key-scoped calls in
apps/server/src/obs/plugin.routes.ts: GET /api/obs/:slug/slots and
POST /api/obs/:slug/token.

Three pieces, all in core/ with no OBS dependency:

- stplugin::json -- a small, strict JSON reader. Hand-rolled rather than
  vendoring nlohmann because the only JSON this plugin ever sees is two
  fixed-shape responses from its own server, and the parser has to build on
  three platforms with no package-manager step in CI. It never throws,
  bounds its recursion (kMaxDepth=32) so a hostile response cannot overflow
  the stack inside OBS, rejects trailing garbage, and returns the caller's
  fallback for wrong-typed access instead of aborting.

- stplugin::HttpClient -- a two-method injectable interface, with libcurl
  behind it on Linux/macOS and WinHTTP on Windows. WinHTTP rather than curl
  on Windows because it ships with the OS and does TLS through SChannel: the
  self-hosted winvm-builder runner has no package manager, and per the
  scaffold README does not even have cmake preinstalled. Both backends cap
  the response body at 4 MiB, keep TLS verification on (the read key is a
  credential), and honour a whole-request timeout.

- stplugin::ApiClient -- maps the responses onto an ApiStatus enum that
  distinguishes NotFound (404), Unavailable (503), NetworkError,
  MalformedResponse and InvalidConfig. It deliberately does not claim to
  know whether a 404 was a wrong key or an unknown slug, because the server
  deliberately does not say. Server URLs are normalised the way an operator
  actually pastes them, defaulting to https so the read key is never sent in
  the clear by accident, and redactedUrl() exists so a URL can be logged
  without the key.

Tests (279 checks across two new suites) run at two levels: a fake
HttpClient covering every response and error branch, and a real loopback
HTTP server on 127.0.0.1 driving the actual platform backend -- so libcurl
on Linux/macOS and WinHTTP on Windows are each exercised in CI rather than
assumed. The loopback cases deliberately include the ones that must not hang
OBS: a truncated JSON body, a connection accepted and closed without a
reply, non-HTTP garbage, a dead port, and a stalled server that has to be
cut off by the client's own timeout.

Verified locally on Ubuntu 24.04:
  ctest --test-dir build --output-on-failure -> 4/4 passed
  test_json: 158 checks passed
  test_api_client: 121 checks passed

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RL8abRmgFXkVASHkkqiJbE

core/CMakeLists.txt

@@ -4,10 +4,23 @@
 # 2026-09-06-obs-camera-plugin-design.md in the streamer-tools repo) --
 # this must build and test headlessly on every platform.
 
-add_library(stplugin_core STATIC
+set(STPLUGIN_CORE_SOURCES
     src/core.cpp
+    src/json.cpp
+    src/http_common.cpp
+    src/api_client.cpp
 )
 
+# HTTP backend, one per platform. See core/include/stplugin/http.h for why
+# this is split rather than using libcurl everywhere.
+if(WIN32)
+    list(APPEND STPLUGIN_CORE_SOURCES src/http_winhttp.cpp)
+else()
+    list(APPEND STPLUGIN_CORE_SOURCES src/http_curl.cpp)
+endif()
+
+add_library(stplugin_core STATIC ${STPLUGIN_CORE_SOURCES})
+
 target_include_directories(stplugin_core
     PUBLIC
         ${CMAKE_CURRENT_SOURCE_DIR}/include
@@ -18,6 +31,16 @@ target_link_libraries(stplugin_core
         LiveKit::livekit
 )
 
+if(WIN32)
+    target_link_libraries(stplugin_core PRIVATE winhttp)
+else()
+    find_package(CURL REQUIRED)
+    target_link_libraries(stplugin_core PRIVATE CURL::libcurl)
+endif()
+
+find_package(Threads REQUIRED)
+target_link_libraries(stplugin_core PUBLIC Threads::Threads)
+
 set_target_properties(stplugin_core PROPERTIES
     POSITION_INDEPENDENT_CODE ON
 )