Add proximity radar visualization and signal history heatmap

Backend:
- Add device_key.py for stable device identification (identity > public MAC > fingerprint)
- Add distance.py with DistanceEstimator class (path-loss formula, EMA smoothing, confidence scoring)
- Add ring_buffer.py for time-windowed RSSI observation storage
- Extend BTDeviceAggregate with proximity_band, estimated_distance_m, distance_confidence, rssi_ema
- Add new API endpoints: /proximity/snapshot, /heatmap/data, /devices/<key>/timeseries
- Update TSCM integration to include new proximity fields

Frontend:
- Add proximity-radar.js: SVG radar with concentric rings, device dots positioned by distance
- Add timeline-heatmap.js: RSSI history grid with time buckets and color-coded signal strength
- Update bluetooth.js to initialize and feed data to new components
- Replace zone counters with radar visualization and zone summary
- Add proximity-viz.css for component styling

Tests:
- Add test_bluetooth_proximity.py with unit tests for device key stability, EMA smoothing,
  distance estimation, band classification, and ring buffer functionality

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

utils/bluetooth/constants.py

@@ -120,6 +120,63 @@ RANGE_NEARBY = 'nearby'
 RANGE_FAR = 'far'
 RANGE_UNKNOWN = 'unknown'
 
+# =============================================================================
+# PROXIMITY BANDS (new visualization system)
+# =============================================================================
+
+PROXIMITY_IMMEDIATE = 'immediate'  # < 1m
+PROXIMITY_NEAR = 'near'           # 1-3m
+PROXIMITY_FAR = 'far'             # 3-10m
+PROXIMITY_UNKNOWN = 'unknown'
+
+# RSSI thresholds for proximity band classification (dBm)
+PROXIMITY_RSSI_IMMEDIATE = -40  # >= -40 dBm -> immediate
+PROXIMITY_RSSI_NEAR = -55       # >= -55 dBm -> near
+PROXIMITY_RSSI_FAR = -75        # >= -75 dBm -> far
+
+# =============================================================================
+# DISTANCE ESTIMATION SETTINGS
+# =============================================================================
+
+# Path-loss exponent for indoor environments (typical range: 2-4)
+DISTANCE_PATH_LOSS_EXPONENT = 2.5
+
+# Reference RSSI at 1 meter (typical BLE value)
+DISTANCE_RSSI_AT_1M = -59
+
+# EMA smoothing alpha (higher = more responsive, lower = smoother)
+DISTANCE_EMA_ALPHA = 0.3
+
+# Variance thresholds for confidence scoring (dBm^2)
+DISTANCE_LOW_VARIANCE = 25.0    # High confidence
+DISTANCE_HIGH_VARIANCE = 100.0  # Low confidence
+
+# =============================================================================
+# RING BUFFER SETTINGS
+# =============================================================================
+
+# Observation retention period (minutes)
+RING_BUFFER_RETENTION_MINUTES = 30
+
+# Minimum interval between observations per device (seconds)
+RING_BUFFER_MIN_INTERVAL_SECONDS = 2.0
+
+# Maximum observations stored per device
+RING_BUFFER_MAX_OBSERVATIONS = 1000
+
+# =============================================================================
+# HEATMAP SETTINGS
+# =============================================================================
+
+# Default time window for heatmap (minutes)
+HEATMAP_DEFAULT_WINDOW_MINUTES = 10
+
+# Default bucket size for downsampling (seconds)
+HEATMAP_DEFAULT_BUCKET_SECONDS = 10
+
+# Maximum devices to show in heatmap
+HEATMAP_MAX_DEVICES = 50
+
 # =============================================================================
 # COMMON MANUFACTURER IDS (OUI -> Name mapping for common vendors)
 # =============================================================================