feat: search-engine indexing — robots.txt, sitemap.xml, llms.txt, SEO meta

robots.txt (sitemap pointer, /metrics and /scrape/ disallowed),
sitemap.xml (/ and /docs), and llms.txt (site + API summary for LLM
crawlers) served at the domain root. Dashboard head gains title with
keywords, meta description, canonical, Open Graph/Twitter tags,
schema.org WebApplication JSON-LD, theme-color, and an inline SVG
favicon (also silences the /favicon.ico 404).

tests/test_dashboard.py

@@ -40,6 +40,26 @@ def test_dashboard_loads_additional_thaiwater_sensors():
     assert "applyStationSearch" in html
 
 
+def test_dashboard_has_seo_and_indexing_files():
+    html = DASHBOARD_PATH.read_text(encoding="utf-8")
+    static_dir = DASHBOARD_PATH.parent
+
+    assert '<meta name="description"' in html
+    assert '<link rel="canonical" href="https://water.buildfor.life/">' in html
+    assert 'property="og:title"' in html
+    assert 'rel="icon"' in html
+
+    robots = (static_dir / "robots.txt").read_text(encoding="utf-8")
+    assert "Sitemap: https://water.buildfor.life/sitemap.xml" in robots
+    assert "<loc>https://water.buildfor.life/</loc>" in (static_dir / "sitemap.xml").read_text(encoding="utf-8")
+    assert "Ping River Live Monitor" in (static_dir / "llms.txt").read_text(encoding="utf-8")
+
+    from src import web_api
+
+    routes = {route.path for route in web_api.app.routes}
+    assert {"/robots.txt", "/llms.txt", "/sitemap.xml"} <= routes
+
+
 def test_dashboard_shows_hii_rainfall_layer():
     html = DASHBOARD_PATH.read_text(encoding="utf-8")