Defer V3-08/V3-09, add V3-17 to investigate self-hosted OSRM

Per request: V3-08's routing-engine direction is decided (self-hosted OSRM over a hosted API or the public demo instance) but standing one up is split into its own ticket, V3-17, since provisioning a server is different work from the app code that calls it. V3-08 and V3-09 are marked Deferred pending V3-17.

V3-17 also names a real tension worth flagging rather than quietly ignoring: this backlog's own organizing principle is 'v3 is everything buildable with no server,' and a self-hosted OSRM instance is a server -- so V3-08/V3-09 sit oddly under docs/v3/ once V3-17 exists. Left unresolved by design (a documentation call for later), noted in both V3-17 and BACKLOG.md's v4 section rather than silently renumbering tickets that are already cross-referenced throughout the docs.

docs/BACKLOG.md

@@ -252,6 +252,14 @@ Split out from v3 deliberately. These three are **one programme, not three items
 needs a server, an identity system, and a privacy stance, and none of them is worth
 building alone. Nothing in v3 depends on any of them.
 
+**A fourth item that doesn't fit that programme but shares its precondition:**
+self-hosted road-snapped routing (v3's V3-08/V3-09, gated on
+[V3-17](v3/V3-17-osrm-hosting.md)). It needs a server the same way this section's three
+items do, but nothing about identity, privacy, or the group-ride use case — it's routing
+infrastructure, not a rider-facing programme. Filed under `docs/v3/` for now since that's
+where the tickets it unblocks already live; flagged here because "v3 is everything
+buildable with no server" stops being strictly true the moment V3-17 is picked up.
+
 ## Live group rides
 
 Invite someone to a ride and see both of you on the map, live.