feat: dashboard token auth + README v2/v3 payload sections split

Dashboard:
- Optional DASHBOARD_TOKEN env var (K8s secret 'snek-dashboard-secret')
- Flask before_request check via Authorization header or ?token= param
- /webhook and /healthz remain public (SNEK POST needs to reach webhook)
- Frontend prompts for token on first load, stores in localStorage
- Token forwarded on fetch() calls and EventSource URL

README decoding section:
- Split into two full sub-sections for Sens'it Discovery (v3) and Sens'it v2
- Explicit bit ordering note (MSB-LSB v3 vs LSB-MSB v2)
- Byte-by-byte tables, formulas per mode, worked examples
- Note that auto-detection uses byte 0 bits 2-0 == 0b110 marker for v3

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

deploy/dashboard.yaml

@@ -38,6 +38,12 @@ spec:
           env:
             - name: DB_PATH
               value: /data/messages.db
+            - name: DASHBOARD_TOKEN
+              valueFrom:
+                secretKeyRef:
+                  name: snek-dashboard-secret
+                  key: token
+                  optional: true
           volumeMounts:
             - name: data
               mountPath: /data