feat(server): initial implementation

This commit captures the full server code accumulated across
several development sessions. It includes the following layers,
applied in roughly this order during development:

1. Base infrastructure
   - cmd/poc-server HTTP+WebSocket server, /health, /stats
   - internal/transport wire types (Envelope + all message types)
   - internal/catalog, internal/realtime, internal/stats
   - internal/storage/postgres with migrations 001-004
   - .env, .env.example, docker-compose, Dockerfile, scripts/

2. Swagger documentation
   - go get github.com/swaggo/http-swagger
   - swag init produces docs/docs.go
   - /swagger/index.html UI, /swagger/doc.json spec
   - annotations on health/stats/ws and catalog handlers

3. Races API
   - internal/races/{store,service,keyset,types}.go
   - migration 005_races.sql: finished_races, race_plans, race_queue
   - GET /api/races with keyset pagination, status filter
   - GET /api/races/upcoming
   - POST /api/races/queue/join, GET, DELETE
   - POST/GET /api/races/plans, DELETE /{id}
   - lobby.RaceMeta simplification (no host_id/host_name)

4. Races seeder
   - internal/races/seed with deterministic generator
   - --seed-races / --reset CLI flags in main.go
   - 30 finished + 5 live + 5 plans + 4 queue entries

5. Drivers and clans
   - internal/drivers, internal/clans (CRUD, validation)
   - migration 008_drivers_clans.sql
   - /api/clans and /api/drivers endpoints
   - 3-letter uppercase nickname/tag validation
   - lobby.DriverMeta: Nickname, AvatarURL, ClanID, ClanTag

6. Podium
   - internal/transport.RacePodiumEntry
   - lobby.SetDriverProfile for in-memory metadata sync
   - migration 007_podium.sql (podium JSONB column)
   - seeder populates top-3 per finished race

7. Live races persistence
   - migration 009_live_persistence.sql: live_races,
     live_race_drivers, lobby_drivers
   - internal/races/live_store.go: LiveStore with write-side
     mirror for lobby.Service mutations
   - Service.collectLive and Upcoming read from Postgres
   - main.go RestoreFromDB rehydrates lobby on startup

8. Unify live + finished into one races table
   - migration 010_unify_races.sql: rename finished_races to
     races, expand status CHECK, merge live rows
   - PgStore now hosts both write paths; LiveStore is a
     thin facade implementing lobby.Persistence
   - seeder resetAll drops only finished/cancelled rows and
     race_plans / race_queue / lobby_drivers

Each layer is consistent on its own; cross-layer changes are
visible in the file history. A future refactor may split this
commit into the per-stage boundaries listed above.

server/internal/races/keyset.go

@@ -0,0 +1,57 @@
+package races
+
+import (
+	"encoding/base64"
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+// Cursor is a keyset cursor: opaque to the client, encoded as
+// base64url("<ms>:<id>") by Encode. Order: descending by ms, then by id.
+//
+// Empty cursor = "start from the newest".
+type Cursor struct {
+	Ms int64
+	ID string
+}
+
+// String returns a human-readable representation (for debug logs).
+func (c Cursor) String() string {
+	return fmt.Sprintf("%d:%s", c.Ms, c.ID)
+}
+
+// IsZero reports whether the cursor is the zero value.
+func (c Cursor) IsZero() bool {
+	return c.Ms == 0 && c.ID == ""
+}
+
+// Encode renders the cursor for an HTTP query string.
+func (c Cursor) Encode() string {
+	if c.IsZero() {
+		return ""
+	}
+	raw := c.String()
+	return base64.RawURLEncoding.EncodeToString([]byte(raw))
+}
+
+// DecodeCursor parses a cursor produced by Cursor.Encode.
+// Returns the zero cursor on empty input.
+func DecodeCursor(s string) (Cursor, error) {
+	if s == "" {
+		return Cursor{}, nil
+	}
+	raw, err := base64.RawURLEncoding.DecodeString(s)
+	if err != nil {
+		return Cursor{}, fmt.Errorf("%w: bad cursor encoding: %v", ErrInvalidInput, err)
+	}
+	parts := strings.SplitN(string(raw), ":", 2)
+	if len(parts) != 2 {
+		return Cursor{}, fmt.Errorf("%w: cursor must be <ms>:<id>", ErrInvalidInput)
+	}
+	ms, err := strconv.ParseInt(parts[0], 10, 64)
+	if err != nil {
+		return Cursor{}, fmt.Errorf("%w: cursor ms: %v", ErrInvalidInput, err)
+	}
+	return Cursor{Ms: ms, ID: parts[1]}, nil
+}