Break v3 into 16 tickets

One file per feature, in the v2 shape that worked: goal, context, design,
implementation, acceptance criteria, tests, risks, out of scope -- written before
implementing so the risks are on paper rather than walked into.

Three carry more weight than their size suggests. V3-01 ships the port's first
real migration, and since the destructive fallback is gone, getting addColumn and
a v1-database test right matters more than the feature. V3-08 forces a
routing-engine decision with ongoing cost, so it sits behind a RoutingService
interface mirroring what LocationSource did for GPS. V3-13 is not code at all --
it answers the three questions open since v2, and V3-15 may close unbuilt as a
result, which is a legitimate outcome.

Several tickets record constraints that are easy to lose: no activity picker in
front of Start, because the founding premise is press-and-go with gloves; the
Route-to-Trip foreign key must not cascade, or deleting an old plan deletes the
ride; V3-14 will deliberately break the parity harness by adding GPX <type>, and
that expectation should be updated rather than the check dropped; and V3-16 must
not regress the explicit text colours that exist because of the black-on-black bug.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

docs/BACKLOG.md

@@ -18,6 +18,9 @@
 Everything known-outstanding, with enough context to pick up cold. Nothing here is
 committed to — it is a menu, roughly ordered by value.
 
+**v3 is now broken out into tickets: [v3/README.md](v3/README.md).** This document stays
+as the reasoning; the tickets are the executable form.
+
 **v3 is everything that can be built with no server.** **v4 is everything that cannot.**
 That split is the most useful thing in this document: it means v3 can proceed indefinitely
 without anyone deciding to run infrastructure or hold other people's location data.