Add FM ANTCAP antenna calibration: HTTP sweep control + EEPROM persistence

Confirmed live tonight that the Si4684's automatic front-end tuning
(FE_VARM/VARB properties) is measurably suboptimal on this board: a
129-value ANTCAP sweep (0-128, AN851 Appendix A) at four different FM
frequencies found antcap=102 beats auto-tune by +6 to +11 dB RSSI and
+3 to +11 dB SNR everywhere tested, consistent with the front-end
network component mismatch already logged in
docs/si4684-rf-investigation-report.md — the board's actual matching
network differs from the AN851 reference network those auto-tune
constants were derived from, so a fixed empirical override
compensates for a gap the chip's own algorithm can't see.

Wiring, bottom to top:
- Si4684Driver::tuneFm() already took an antCap byte; threaded it
  through core::ITuner::tuneFm() and si4684::Si4684Tuner::tuneFm() as
  a new parameter (default 0 = auto, unchanged behaviour for every
  existing caller).
- TunerService::tuneFm() takes an optional override instead: omitted,
  it falls back to a new defaultFmAntCap_ member so every ordinary FM
  tune (seek, scan, station recall, live UI) benefits automatically
  once calibrated, not just calls that pass antcap explicitly.
- POST /api/tuner/tune gained an optional "antcap" field for sweeping
  live without touching the saved calibration.
- POST /api/tuner/calibrate-antenna commits a sweep result: writes it
  to the 24AA025E48 EEPROM's user-writable region (word address 0x00,
  separate from the factory-locked EUI-48 at 0xFA-0xFF) via a new
  Eeprom24aa::writeFmAntCap()/readFmAntCap() pair, and immediately
  updates the live TunerService default — no reboot needed to take
  effect, though HardwareBootstrap::boot() also loads it at every
  boot so it survives power cycles. Same net::AntennaCalibration
  function-pointer bridge pattern as PhoneStreamSink/BleProvisioning,
  so components/net stays free of eeprom24aa headers.

Verified end to end on hardware: swept and found 102, saved it via
the new endpoint, confirmed the live default changed immediately,
then power-cycled and confirmed the boot log reports "FM ANTCAP
calibration loaded: 102" and a subsequent default tune reflects it.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0178rASQ6ZETPMUamvpoR2KR

Software/components/services/tuner/include/tuner/TunerService.hpp

@@ -87,6 +87,10 @@ public:
      *
      * @dname    tuneFm
      * @param    frequency  Validated FM centre frequency.
+     * @param    antCap     Front-end antenna varactor override for this one
+     *                      tune (e.g. for a calibration sweep). Omit to use
+     *                      defaultFmAntCap_ (the board's saved calibration,
+     *                      or hardware auto-tune if never calibrated).
      * @return   Ok on success, or a TunerError from ITuner.
      * @pubstate writes lastFmFrequency_ on success.
      *
@@ -94,7 +98,22 @@ public:
      * @date     2026-07-06
      */
     [[nodiscard]] std::expected<void, core::TunerError> tuneFm(
-        core::FrequencyKHz frequency);
+        core::FrequencyKHz frequency,
+        std::optional<std::uint8_t> antCap = std::nullopt);
+
+    /**
+     * @brief    setDefaultFmAntCap — set the board's calibrated ANTCAP.
+     *
+     * @dname    setDefaultFmAntCap
+     * @param    antCap  Value applied to every FM tune that doesn't pass an
+     *                   explicit override (0 = chip auto-tune, the factory
+     *                   default before any calibration is saved).
+     * @pubstate writes defaultFmAntCap_. Does not itself re-tune.
+     *
+     * @author   Michele Bigi
+     * @date     2026-08-19
+     */
+    void setDefaultFmAntCap(std::uint8_t antCap) noexcept;
 
     /**
      * @brief    seekFm — seek FM in the given direction.
@@ -200,6 +219,7 @@ private:
     std::uint8_t lastDabIndex_;
     core::FrequencyKHz lastFmFrequency_;
     std::uint8_t volume_;
+    std::uint8_t defaultFmAntCap_;
     std::optional<std::uint32_t> lastPlayedServiceId_;
     std::optional<std::uint32_t> lastPlayedComponentId_;
 };

Software/components/services/tuner/src/TunerService.cpp

@@ -138,9 +138,15 @@ TunerService::TunerService(core::ITuner& tuner)
     , lastDabIndex_(0U)
     , lastFmFrequency_(defaultFmFrequency())
     , volume_(40U)
+    , defaultFmAntCap_(0U)
 {
 }
 
+void TunerService::setDefaultFmAntCap(std::uint8_t antCap) noexcept
+{
+    defaultFmAntCap_ = antCap;
+}
+
 std::expected<core::TunerStatus, core::TunerError> TunerService::refreshStatus()
 {
     auto status = tuner_.readStatus();
@@ -167,9 +173,10 @@ std::expected<void, core::TunerError> TunerService::tuneDab(
 }
 
 std::expected<void, core::TunerError> TunerService::tuneFm(
-    core::FrequencyKHz frequency)
+    core::FrequencyKHz frequency, std::optional<std::uint8_t> antCap)
 {
-    if (auto result = tuner_.tuneFm(frequency); !result) {
+    if (auto result = tuner_.tuneFm(frequency, antCap.value_or(defaultFmAntCap_));
+        !result) {
         return result;
     }
     lastFmFrequency_ = frequency;