feat: Nagercoil single-rider assignment endpoint

Adds POST /api/v1/optimization/nagercoil/riderassign. Nagercoil runs with one
rider, so this skips the matching/VRP/pattern logic in riderassign and straight-
assigns every order to the single rider, then reuses the shared TSP sequencer
(_sync_optimize_route) so stop order, cumulative kms and per-stop ETAs match the
main endpoint's response shape.

Rider defaults to Sivakumar Subramani (userid 852, 8248176099) via the
NAGERCOIL_RIDER constant; a `rider` object in the request body overrides it.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018jhRcey2fxhwz2SuPVm7c5

app/routes/__init__.py

@@ -7,6 +7,7 @@ from .ml_admin import router as ml_router, web_router as ml_web_router
 from .batch_analytics import router as batch_analytics_router
 from .riders import router as riders_router
 from .doormile import router as doormile_router
+from .nagercoil import router as nagercoil_router
 
 __all__ = [
     "optimization_router",
@@ -17,4 +18,5 @@ __all__ = [
     "batch_analytics_router",
     "riders_router",
     "doormile_router",
+    "nagercoil_router",
 ]