Scope dial v2: Nearby / Region / Country / World radius on the homepage

Codex-approved evolution: the reader controls the "emotional radius" of the landing.

- Census-region "Regional" grain (geo.region_of / region_states). Scope-aware tiering
  (queries.home_tiers): closest->widest lead, confidence-gated on state + region, never
  a hard filter — blends outward so the set is always full. 'world' = the global brief.
- queries.home_brief takes a scope; /api/brief gains a scope param (nearby|region|
  country|world). Country-only / non-US homes collapse to country.
- Homepage dial replaces the 2-button toggle: adaptive stops (4 with a US state, else
  Country/World), persisted scope, "Good news closest first" framing. Concrete, soft
  section labels (Around New Jersey / Across the Northeast / Across the US / Around the
  world) so the reader sees the dial worked.

Backend 366 + frontend tests green. (Latest feed still on v1 local-first; aligning it
to the dial is the immediate follow-up.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

goodnews/geo.py

@@ -38,6 +38,34 @@ US_STATES = {
     "district of columbia": "DC", "washington dc": "DC", "washington d c": "DC",
 }
 
+# US Census Bureau regions — the "Regional" grain for the scope dial. Standard,
+# explainable, not arbitrary. DC sits in the South (South Atlantic) per Census.
+US_REGIONS = {
+    "Northeast": {"CT", "ME", "MA", "NH", "RI", "VT", "NJ", "NY", "PA"},
+    "Midwest": {"IL", "IN", "MI", "OH", "WI", "IA", "KS", "MN", "MO", "NE", "ND", "SD"},
+    "South": {"DE", "FL", "GA", "MD", "NC", "SC", "VA", "DC", "WV", "AL", "KY", "MS",
+              "TN", "AR", "LA", "OK", "TX"},
+    "West": {"AZ", "CO", "ID", "MT", "NV", "NM", "UT", "WY", "AK", "CA", "HI", "OR", "WA"},
+}
+
+
+def region_of(state_code: str | None) -> str | None:
+    """The Census region name containing a US state code, or None."""
+    if not state_code:
+        return None
+    sc = state_code.upper()
+    for name, states in US_REGIONS.items():
+        if sc in states:
+            return name
+    return None
+
+
+def region_states(state_code: str | None) -> list[str]:
+    """Sorted state codes in the same Census region as a state (incl. it)."""
+    name = region_of(state_code)
+    return sorted(US_REGIONS[name]) if name else []
+
+
 # Common countries + aliases (extensible). Anything not here returns None -> we drop
 # the country rather than store garbage. breadth still captures national/global, etc.
 COUNTRY_TO_ISO = {