feat: surface LN1/LN2 (L1/L10) percentiles through SLMM

Completes the SLMM side of the L1/L10 live-display contract. The NL-43's
DOD response carries percentile slots LN1-LN5 (channel 1, parts[5]/[6]);
parse the first two and expose them as ln1/ln2 end to end:

- NL43Snapshot dataclass: ln1/ln2 fields
- NL43Status model: ln1/ln2 columns (+ migrate_add_ln_percentiles.py)
- DOD parser: snap.ln1=parts[5], snap.ln2=parts[6]
- persist_snapshot writes them
- all /status data dicts, StatusPayload, and the DRD stream payload emit
  ln1/ln2 (null on the DRD stream itself, which doesn't carry percentiles)

Labels: device LN1 defaults to L5, not L1 — Terra-View defaults the label
to L1/L10, so the device's Ln1/Ln2 slots must be set to 1%/10% for the
labels to be accurate (dynamic label emission is a follow-up).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/models.py

@@ -41,6 +41,8 @@ class NL43Status(Base):
     lmax = Column(String, nullable=True)  # Maximum level
     lmin = Column(String, nullable=True)  # Minimum level
     lpeak = Column(String, nullable=True)  # Peak level
+    ln1 = Column(String, nullable=True)  # Percentile slot LN1 (configurable; device default L5, contract L1)
+    ln2 = Column(String, nullable=True)  # Percentile slot LN2 (configurable; device default L10)
     battery_level = Column(String, nullable=True)
     power_source = Column(String, nullable=True)
     sd_remaining_mb = Column(String, nullable=True)