Add intelligent caching layer for SFM device data

Introduces sfm/cache.py — a SQLite-backed cache (via SQLAlchemy) that
sits between the SFM REST endpoints and the device, eliminating redundant
cellular downloads for data that doesn't change.

Cache behaviour by data type:
- Device info / compliance config: cached until a config write occurs;
  POST /device/config now calls mark_config_dirty() to force a fresh read
  on the next /device/info call.
- Event headers + peak values: cached permanently (append-only). On
  subsequent calls to /device/events, the server does a fast count_events()
  (~2s) instead of a full download (~10-30s); only new events are fetched
  from the device and merged into the cache.
- Full waveforms (raw ADC samples): cached permanently — immutable once
  recorded. Repeated requests for the same waveform return instantly with
  zero device contact.
- Monitor status (battery, memory, is_monitoring): 30-second TTL; auto-
  invalidated on start/stop monitoring commands.

All endpoints gain a ?force=true param to bypass the cache when needed.
New endpoints: GET /cache/stats, DELETE /cache/device.
Adds requirements.txt listing fastapi, uvicorn, sqlalchemy, pyserial.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

requirements.txt

@@ -0,0 +1,4 @@
+fastapi
+uvicorn
+sqlalchemy
+pyserial