openspec: revise route-surface-breakdown — add async Overpass backfill + SSE

Builds on the merged A-only proposal. Broadens the capability:
- breakdown now on routes AND activities;
- Path 1 (sync) unchanged: BRouter waytags → breakdown at Planner save;
- Path 2 (async): a `surface-backfill` pg-boss job map-matches geometry to OSM
  via Overpass for imports/uploads/older rows, stores the breakdown, and pushes
  a `surface_breakdown` SSE event so an open detail page fills in live;
- privacy note: backfill sends geometry to Overpass → route via the proxy /
  self-hostable Overpass; documented in the manifest.

Validates with `openspec validate --strict`. Specs only; no code.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

openspec/changes/route-surface-breakdown/proposal.md

@@ -1,46 +1,57 @@
 ## Why
 
-"How much of this is gravel?" is a top-three question for anyone choosing a
-route — Komoot's surface/waytype breakdown bars are one of its most-used
-features, and `@trails-cool/map-core` already ships the surface/highway colour
-palettes. We also already *compute* accurate per-segment surface and highway
-tags from BRouter while planning (`route-merge` → `EnrichedRoute.surfaces[]`),
-then **throw them away on save**. This change captures that data and shows it.
+"How much of this is gravel?" is a top-three question for choosing a route, and
+Komoot's surface/waytype bars are one of its most-used features.
+`@trails-cool/map-core` already ships the surface/highway colour palettes, and
+the Planner already *computes* accurate per-segment surface/highway from BRouter
+(`route-merge` → `EnrichedRoute.surfaces[]`) — then **discards it on save**.
+This change captures that data, shows it as proportion bars, and **backfills
+everything else** (imports, uploads, older rows) asynchronously so the bars
+aren't limited to Planner routes.
 
 ## What Changes
 
-- On **route detail**, show surface and waytype **proportion bars** (e.g.
-  "62% asphalt · 28% gravel · 10% path"), coloured with the `map-core`
-  `SURFACE_COLORS` / `HIGHWAY_COLORS` palettes, with per-segment km/percent.
-- **Persist the breakdown for Planner-created routes**: compute a
-  distance-weighted surface + waytype distribution from the BRouter waytags at
-  save time, send it in the planner→journal handoff, and store it on the route.
-- Degrade gracefully: routes without breakdown data (older routes, or activities
-  / manually-imported GPX that never had waytags) simply don't show the bars.
+- On **route and activity detail**, show surface and waytype **proportion bars**
+  (e.g. "62% asphalt · 28% gravel · 10% path"), coloured with the `map-core`
+  `SURFACE_COLORS` / `HIGHWAY_COLORS` palettes, with per-category km/percent.
+- **Path 1 — synchronous (Planner routes):** compute a distance-weighted
+  surface + waytype distribution from the BRouter waytags at save time, send it
+  in the planner→journal handoff, store it. Accurate, free, no external call.
+- **Path 2 — async backfill (everything else):** for a route/activity that has
+  geometry but no breakdown (imports, manual uploads, pre-existing rows), a
+  background job map-matches the geometry to OSM ways via Overpass, computes the
+  breakdown, stores it, and **pushes an SSE event** so an open detail page fills
+  the bars in live.
+- Degrade gracefully: no geometry / not-yet-backfilled → no bars (or a
+  "computing…" state while a backfill is in flight).
 
 ## Capabilities
 
 ### New Capabilities
 
-- `route-surface-breakdown`: the surface/waytype proportion bars on route
-  detail, how the distribution is derived from BRouter waytags, persisted at
-  save, and rendered.
+- `route-surface-breakdown`: the surface/waytype proportion bars on route &
+  activity detail — derived synchronously from BRouter waytags (Planner) or
+  asynchronously via an Overpass backfill job with an SSE live-update.
 
 ### Modified Capabilities
 
-<!-- `planner-journal-handoff` gains one optional field in its save payload;
-     noted in design, but the breakdown is a new capability, not a change to the
-     handoff's existing requirements. -->
+<!-- `planner-journal-handoff` gains one optional payload field; `background-jobs`
+     and `sse-broker` gain a new job + event, but those are additive uses of
+     existing transports, owned by this new capability. -->
 
 ## Impact
 
-- **Schema**: a nullable `surface_breakdown` jsonb column on `journal.routes`
-  (additive — `pnpm db:push`, no data migration).
-- **Planner**: `api.save-to-journal` computes the distance-weighted breakdown
-  from the session `EnrichedRoute` (surfaces/highways + coordinates) and adds it
-  to the POST body.
-- **Journal**: the route callback (`api.routes.$id.callback`) accepts + stores
-  it; the route detail loader exposes it; a `SurfaceBreakdown` component renders
-  the bars (reusing `map-core` palettes + `stats.ts` distance formatting).
-- **No new external dependency** — BRouter already returns the tags; no Overpass
-  call. Imported activities/uploads are out of scope for v1 (see design).
+- **Schema**: nullable `surface_breakdown` jsonb on `journal.routes` **and**
+  `journal.activities` (additive — `pnpm db:push`).
+- **Planner**: `api.save-to-journal` computes + sends the breakdown.
+- **Journal**:
+  - route callback stores the synchronous breakdown;
+  - a typed `surface-backfill` pg-boss job (Overpass map-match → breakdown →
+    store → SSE), enqueued after imports/uploads and available as a sweep;
+  - a journal-side Overpass way/surface client (reusing the existing
+    `/api/overpass` proxy pattern);
+  - an SSE `surface_breakdown` event + a detail-page subscription;
+  - a `SurfaceBreakdown` component (reuses `map-core` palettes + `stats.ts`).
+- **i18n**: `journal.surface.*` (en + de).
+- **Privacy**: the backfill sends route geometry to Overpass — see design for
+  the proxy/self-host + visibility considerations.