Step 2.2: config registry and NVS store, GET/POST /api/config

- aes67_web/cfg.c: cfg_register(group, defaults, validate, apply),
  cfg_override_defaults() for project defaults, cfg_get(). Stored values
  are loaded on register and merged over defaults (known keys, matching
  types). NVS keeps only keys that differ from defaults, one JSON string
  per group, so new firmware defaults still reach untouched settings.
- POST /api/config: type check against defaults, per-group validation,
  nothing stored unless every group passes; 400 "group.key: reason".
  Unknown keys/groups are ignored. Returns the new config.
- Core groups registered by their components with the UI's ranges:
  ptp (aes67_ptp), aes67 (aes67_tx, incl. multicast range and mono_sum
  => 1 channel), net/inet (aes67_net), log (aes67_syslog).
- Project: PROJECT.def overrides (hostname p4-aes67, name P4 AES67) and
  the "source" group in main/project_cfg.c.
- Config is stored only; nothing is applied live yet.
- Verified on board: defaults, valid save, 8 rejected values (nothing
  stored), persistence across reboot, restore to defaults.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

main/project_cfg.c

@@ -0,0 +1,33 @@
+// Project config: defaults over the core groups and the "source" group.
+#include "project_cfg.h"
+
+#include "aes67_cfg.h"
+#include "esp_err.h"
+
+static const char SOURCE_DEFAULTS[] =
+    "{\"mode\":\"spotify\",\"spotify_name\":\"P4 AES67\",\"spotify_bitrate\":320,\"hls_url\":\"\","
+    "\"autoplay\":false,\"gain_db\":0,\"failover_delay_s\":5,\"failover_on_pause\":false}";
+
+static bool source_validate(const cJSON *g, char *err, size_t n)
+{
+    static const char *const modes[] = { "spotify", "hls", "auto", "off", NULL };
+    static const double bitrates[] = { 96, 160, 320 };
+    return cfg_check_enum(g, "mode", modes, err, n) &&
+           cfg_check_str(g, "spotify_name", 1, 63, err, n) &&
+           cfg_check_num_in(g, "spotify_bitrate", bitrates, 3, err, n) &&
+           cfg_check_str(g, "hls_url", 0, 255, err, n) &&
+           cfg_check_num(g, "gain_db", -60, 12, err, n) &&
+           cfg_check_int(g, "failover_delay_s", 0, 3600, err, n);
+}
+
+void project_cfg_defaults(void)
+{
+    // Same as PROJECT.def in web/index.html.
+    ESP_ERROR_CHECK(cfg_override_defaults("aes67", "{\"name\":\"P4 AES67\"}"));
+    ESP_ERROR_CHECK(cfg_override_defaults("net", "{\"hostname\":\"p4-aes67\"}"));
+}
+
+void project_cfg_register(void)
+{
+    ESP_ERROR_CHECK(cfg_register("source", SOURCE_DEFAULTS, source_validate, NULL));
+}