feat(backend,web): tuned bike custom model on GraphHopper 12

[cafca]

Summary

• Replace the legacy vehicle: bike, weighting: custom profile with a single Add Fathom analytics via configurable build-time env var #12-style custom-model bike profile mounted from backend/custom_models/beebeebike.json.
• Migrate the GraphHopper config to the GH 12 schema: drop vehicle: / weighting:, nest turn_costs: { vehicle_types: [bicycle], u_turn_costs: 20 }, register custom models via custom_models.directory: /custom_models.
• Lower the web "Route directness" default from 70 → 60 to match the model's distance_influence so first-load behavior is consistent (the slider can still override per request).

What the custom model does

Layered on top of GH's built-in bike_priority baseline, it applies these multipliers:

Priority

• Wrong-way oneway enforcement: !bike_access && (!backward_bike_access || roundabout) → 0, with the asymmetric oneway:bicycle=no case allowed at 0.2.
• Access penalties: bike_road_access == PRIVATE → 0.2; road_access FORESTRY/AGRICULTURAL → 0.6, MILITARY → 0.1.
• road_environment FERRY → 0.3, FORD → 0.2.
• Hard block: mtb_rating > 2 → 0 (no T2+ MTB segments).
• Cycle infra boost: cycleway == TRACK → 1.3, cycleway == LANE → 1.1.
• Comfort: get_off_bike → 0.6; surface COBBLESTONE → 0.5, loose surfaces SAND/GROUND/DIRT → 0.7.

Speed (km/h)

• Base: limit_to: bike_average_speed.
• Wrong-way contraflow: limit_to: 6.
• Surface slowdowns: cobblestone × 0.75, paving stones / gravel × 0.9.
• Slope shaping: >6° × 0.7, >12° × 0.5, <−6° × 1.2.

Other

• distance_influence: 60 (slightly more willing to detour than the GH default of 70).
• u_turn_costs: 20 (vs. GH default 10) to discourage tight reroutes.

Notably not in this model: no road_class tuning (TRUNK/PRIMARY/SECONDARY/etc. all use bike_priority as-is), no bike_network boost, no smoothness rules. Those can be added later if needed.

Notes for reviewers

• profiles_ch: []: CH preparation cannot bake custom weighting with dynamic per-request priority rules (rated-area injection, optional distance_influence override). The per-request "ch.disable": true in routing.rs is now redundant but harmless.
• compose.yml gains one bind mount: ./backend/custom_models:/custom_models:ro.
• Mobile API surface unchanged — mobile clients keep sending rating_weight + optional distance_influence, no profile field.
• Any future edit to a profile's priority/speed/distance_influence rules or to turn_costs/u_turn_costs requires wiping the GH cache (data/osm/berlin/graphhopper/) and reimporting; otherwise GH refuses to start with Profile 'bike' does not match.

Test plan

• cargo test --lib (backend, 69 tests)
• cargo clippy --all-targets -- -D warnings
• npm test -- --run (web, 71 tests)
• GraphHopper boots cleanly with the new config and serves /route for the bike profile
• Manual smoke: a route in central Berlin returns geometry and respects oneway directionality