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/v3/V3-08-road-routing.md

@@ -1,6 +1,7 @@
 # V3-08 — Road-snapped routing and ETA
 
-**Phase** Route planning · **Depends on** V3-07, benefits from V3-01 · **Size** L · **Status** Not started
+**Phase** Route planning · **Depends on** V3-07, benefits from V3-01, **blocked on V3-17**
+· **Size** L · **Status** Deferred
 
 ## Goal
 Resolve the actual shortest path along roads between pins, and estimate how long the ride
@@ -11,6 +12,12 @@ This is what Dylan asked for. It is also the ticket with a **decision that canno
 deferred**: road-snapped routing needs a routing engine over OpenStreetMap data, and the
 choice has ongoing consequences.
 
+**Decided, not deferred:** self-hosted OSRM (see the table below). What's actually
+deferred is standing one up — that's [V3-17](V3-17-osrm-hosting.md), split out on
+request because provisioning a server is a different kind of work from building the app
+code that calls it, and this ticket cannot start until V3-17 has something to point
+`RoutingService` at.
+
 ## Design
 
 ### Choosing an engine — decide before writing code