feat: country + region dropdowns for signup — consistent room bucketing

- Replace free-text city/country with cascading country→region selects
- 60+ countries, 400+ pre-defined regions with fixed lat/lng/timezone
- Same region = identical coordinates = same Isha bucket = shared room
- /regions public API endpoint for frontend dropdown data
- auth.py uses pre-defined coords (no geocoding needed for known regions)
- Step 2 validation: both country and city required to proceed

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

Author: Tarteel

backend/api/auth.py

@@ -58,11 +58,17 @@ async def register(body: UserRegisterFull, response: Response, db: AsyncSession
     if result.scalar_one_or_none():
         raise HTTPException(status_code=400, detail="Email already registered")
 
-    # Geocode city
-    geo = await geocode_city(body.city, body.country)
-    if not geo:
-        raise HTTPException(status_code=400, detail=f"Could not geocode '{body.city}, {body.country}'")
-    lat, lng, tz_name = geo
+    # Try pre-defined region coordinates first (fast, reliable, ensures room bucketing)
+    from utils.regions import lookup_region
+    region_coords = lookup_region(body.country, body.city)
+    if region_coords:
+        lat, lng, tz_name = region_coords
+    else:
+        # Fall back to geocoding for any city not in our regions list
+        geo = await geocode_city(body.city, body.country)
+        if not geo:
+            raise HTTPException(status_code=400, detail=f"Could not geocode '{body.city}, {body.country}'")
+        lat, lng, tz_name = geo
 
     user = User(
         email=body.email,

backend/api/regions.py

@@ -0,0 +1,10 @@
+from fastapi import APIRouter
+from utils.regions import get_regions_map
+
+router = APIRouter(prefix="/regions", tags=["regions"])
+
+
+@router.get("", response_model=dict[str, list[str]])
+async def list_regions():
+    """Public endpoint — returns {country: [city, ...]} for the registration dropdowns."""
+    return get_regions_map()

backend/main.py

@@ -15,6 +15,7 @@
 from api.admin import router as admin_router
 from api.friends import router as friends_router
 from api.private_rooms import router as private_rooms_router
+from api.regions import router as regions_router
 from services.scheduler import start_scheduler
 from ws.events import sio
 
@@ -67,6 +68,7 @@ async def lifespan(app: FastAPI):
 app.include_router(admin_router)
 app.include_router(friends_router)
 app.include_router(private_rooms_router)
+app.include_router(regions_router)
 
 # Serve HLS files statically
 hls_dir = Path(settings.HLS_OUTPUT_DIR)

backend/schemas/user.py

@@ -32,6 +32,14 @@ class UserRegisterStep2(BaseModel):
     country: str
     calc_method: int = 3
 
+    @field_validator("city", "country")
+    @classmethod
+    def strip_required(cls, v: str) -> str:
+        v = v.strip()
+        if not v:
+            raise ValueError("This field is required")
+        return v
+
 
 class UserRegisterStep3(BaseModel):
     rakats: int = 8