feat: flood-verdict banner, violet rain palette, per-station bands, chip contrast, Umami

UI/UX review backlog round 2 plus analytics:
- at-a-glance verdict banner (role=status, aria-live) above the tiles:
  ok/watch/danger from P.1 level vs the official 3.70 m stage and the
  24 h forecast, with a not-an-official-warning disclaimer + ThaiWater
  link (also added to the forecast card)
- rainfall dots switch to a violet sequential ramp so they can never be
  confused with flow-status colors; legend gains 'Other markers' rows
  (grey no-data gauges, + bank-height sensors)
- history-chart flood bands now use each station's own /forecast
  thresholds (P.1 official fallback) instead of hardcoded 3.0/4.5
- chipStyle(): contrast-safe text on all code/risk/stage chips (dark ink
  on amber/ochre, darkened fills for white ink); risk/stage ramps drop
  the blue step for a monotonic green-ochre-amber-red escalation
- plain-language tooltips on the flow and capacity tiles
- Umami: client script tag plus opt-out server-side middleware posting
  api-request events (fire-and-forget, 3 s timeout, never blocks)

src/config.py

@@ -87,6 +87,18 @@ class Config:
     )
     HII_BASIN_CODE = int(os.getenv("HII_BASIN_CODE", "6"))  # 6 = Ping Basin
 
+    # Umami analytics (self-hosted). The website id is public (it ships in the
+    # dashboard <script> tag); server-side API tracking posts to /api/send.
+    UMAMI_API_URL = os.getenv("UMAMI_API_URL", "https://stats.buildfor.life/api/send")
+    UMAMI_WEBSITE_ID = os.getenv(
+        "UMAMI_WEBSITE_ID", "00b2be73-8f5f-4400-9029-3be852eb08f7"
+    )
+    UMAMI_TRACK_API = os.getenv("UMAMI_TRACK_API", "true").lower() in (
+        "1",
+        "true",
+        "yes",
+    )
+
     # Scheduler settings
     SCRAPING_INTERVAL_HOURS = int(os.getenv("SCRAPING_INTERVAL_HOURS", "1"))