Fix NL43 DRD field mapping to match official specification

Corrected the parsing of NL43 DRD (Dynamic Range Data) and DOD (Data On Demand)
responses according to the NL43 Communications Guide. The previous implementation
incorrectly mapped d0 (counter field) as a measurement.

Changes:
- Updated DRD/DOD parsing to skip d0 (counter: 1-600)
- Correctly map d1-d5 to lp/leq/lmax/lmin/lpeak measurements
- Added inline documentation referencing DRD format specification
- Included database migration script to revert incorrect field names

DRD format per NL43 spec:
- d0 = counter (1-600) - NOT a measurement
- d1 = Lp (instantaneous sound pressure level)
- d2 = Leq (equivalent continuous sound level)
- d3 = Lmax (maximum level)
- d4 = Lmin (minimum level)
- d5 = Lpeak (peak level)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/routers.py

@@ -652,11 +652,11 @@ async def stream_live(websocket: WebSocket, unit_id: str):
                     "unit_id": unit_id,
                     "timestamp": datetime.utcnow().isoformat(),
                     "measurement_state": snap.measurement_state,
-                    "lp": snap.lp,
-                    "leq": snap.leq,
-                    "lmax": snap.lmax,
-                    "lmin": snap.lmin,
-                    "lpeak": snap.lpeak,
+                    "lp": snap.lp,      # Instantaneous sound pressure level
+                    "leq": snap.leq,    # Equivalent continuous sound level
+                    "lmax": snap.lmax,  # Maximum level
+                    "lmin": snap.lmin,  # Minimum level
+                    "lpeak": snap.lpeak,  # Peak level
                     "raw_payload": snap.raw_payload,
                 })
             except Exception as e: