[coderconnoisseur]

Stacked on #4 (`sync-master-phase1`, the Phase 1 follow-ups synced into `master`). Merge #4 first, then retarget this PR to `master`.

What changed

• Shared event contracts (`pkg/events`): `RideRequestedEvent`, `RidePricedEvent`, `RideMatchedEvent`, `RideUnmatchedEvent` and `Location` defined once, with explicit snake_case JSON tags and a `v` field (version 1). Pricing, matching, assignment and api-gateway import them. Round-trip JSON tests included. This fixes the `RideId` / `RideID` drift in pricing.
• Dead code removed: api-gateway `LogPublisher`, `pricing-service/internal/ports/`, and `matching-service/internal/ports/consumer.go` (an unused consumer interface). Grepped to confirm no imports.
• Gateway validation: `lat`/`lng` are pointer fields, so a valid `0` is accepted. Latitude must be in [-90, 90] and longitude in [-180, 180]; anything else returns 400 and names the field. Handler tests use `httptest` with a fake publisher.
• Health and readiness on api-gateway, pricing, matching and assignment:
  • `GET /health` is liveness and always returns 200 while the process runs.
  • `GET /ready` returns 200 only when dependencies answer, within a 1 s budget. The Kafka check dials the first reachable broker. Matching also checks Redis `PING`. Failing checks are listed in a 503 body.
  • Shared implementation in `pkg/health`, with unit tests using injectable checks.

Deploy note: wire format change

Event JSON keys are now snake_case (`ride_id`, `requested_at`, `matched_at`, `driver_id`, `failed_at`, `surge_multiplier`, `pickup`, `drop`) and every event carries `"v": 1`. Previously keys were Go field names (`RideID`, `RequestedAt`, `MatchedAT`, ...).
Producers and consumers all ship in this PR, so a consistent deploy works. But drain and recreate the `ride.*` topics (or reset consumer-group offsets) before starting the new build, so old and new payloads are not mixed on the same topic.

Not in this PR

• Logical commits: this is a single commit, grouped by the four items above.
• The README and architecture diagram are in the next PR (stacked on this one).