feat(cv): add CVTuning struct and NVS persistence scaffolding

Adds CVTuning to CVState, populates defaults from existing file-scope
constants in cv_init, and introduces config_load_tuning/config_save_tuning
backed by the doorcounter NVS namespace. No runtime behavior change yet;
CV code still reads the existing constants (Task 2 will migrate reads).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

firmware/lib/cv/cv.h

@@ -19,6 +19,15 @@ struct CVTrack {
     int   missed;
 };
 
+struct CVTuning {
+    uint8_t  diff_thresh;   // per-pixel motion threshold
+    int      min_blob_px;   // min foreground pixels for a blob
+    float    max_move;      // max inter-frame track jump (px)
+    int      max_missed;    // frames before drop
+    uint8_t  line_offset;   // 0-100, percent of frame height for virtual line
+    uint32_t cfg_version;   // monotonic; server increments on push
+};
+
 struct CVState {
     uint8_t  background[CV_PIXELS];
     bool     bg_valid;
@@ -28,6 +37,7 @@ struct CVState {
     std::vector<CVTrack> tracks;
     int entries;
     int exits;
+    CVTuning tuning;
 };
 
 struct CVResult {