feat(charging/types): add Bundle, ActiveBundle, and AirtimeBalance types

Adds all data models required for airtime and bundle distribution:

• BundleType enum (Data, Voice, SMS, Hybrid) with as_str()/from_str()
• BundleAllowances (data_bytes, voice_seconds, sms_count, roaming_data_bytes)
• Bundle (bundle_id, name, type, allowances, validity_days, priority,
  amount_unconverted, is_active)
• ActiveBundle (imsi, bundle_id, priority, activated_at, expires_at,
  remaining_allowances) with Redis FromRedisValue/ToRedisArgs impls
• AirtimeBalance (imsi, home_seconds, roaming_seconds, last_updated)
• AirtimeDeduction (deducted, new_balance)

fix(charging): make RatingPlansRepo pool pub(crate) for bundle service access

fix(errors): add BundleNotFound, BundleAlreadyActive, InvalidBundleConfig
variants with correct HTTP status mappings (404, 409, 400)

feat(charging): implement airtime service

Adds airtime management on ChargingEngine via airtime_service.rs:

• add_airtime(imsi, seconds, roaming) — Lua atomic INCRBY on
  Redis airtime:home:{imsi} or airtime:roaming:{imsi}
• get_airtime_balance(imsi) → AirtimeBalance
• deduct_airtime(imsi, seconds, roaming) — Lua atomic check-and-decrby;
  returns InsufficientAirtime if balance < requested
• All operations protected by redis_circuit_breaker

feat(charging): implement bundle service with airtime-funded purchase

Adds bundle lifecycle management on ChargingEngine via bundle_service.rs:

• create_bundle / list_bundles / get_bundle_config / get_bundle_by_id
• activate_bundle_for_subscriber — stores ActiveBundle in Redis with TTL,
  maintains bundle:active:set:{imsi} Redis Set for O(1) lookup,
  writes subscriber_bundles row to PostgreSQL
• consume_from_bundle — priority-ordered deduction from active bundles
• deactivate_bundle — marks bundle inactive in PostgreSQL
• purchase_bundle_with_airtime(imsi, bundle_id) — atomically deducts
  amount_unconverted from airtime balance then activates bundle
• gift_bundle(sender_msisdn, recipient_msisdn, bundle_id) — resolves
  both MSISDNs to IMSIs, deducts from sender airtime, activates on
  recipient
• expire_stale_bundles — background cleanup of expired subscriber_bundles
• record_bundle_activation — Redis audit trail per activation
• All PostgreSQL ops protected by postgres_circuit_breaker

feat(charging): register airtime_service and bundle_service modules

Adds pub mod declarations to apps/charging-engine/src/charging.rs so
rust-analyzer and the compiler can resolve the new files.

feat(handlers): add airtime and bundle HTTP handlers

handlers/airtime.rs:

• POST /v1/airtime/:imsi/add → add_airtime
• GET /v1/airtime/:imsi/balance → get_airtime_balance
• POST /v1/airtime/:imsi/deduct → deduct_airtime

handlers/bundles.rs:

• POST /v1/bundles → create_bundle
• GET /v1/bundles → list_bundles
• GET /v1/bundles/:id → get_bundle
• DELETE /v1/bundles/:id → deactivate_bundle
• POST /v1/bundles/:id/activate → activate_bundle
• GET /v1/bundles/:imsi/active → list_active_bundles
• POST /v1/bundles/:id/purchase → purchase_bundle (by MSISDN, deducts airtime)
• POST /v1/bundles/:id/gift → gift_bundle (sender_msisdn → recipient_msisdn)

Updates handlers/mod.rs with pub mod and pub use for both new modules.

feat(routes): register airtime and bundle routes in protected section

All new routes sit inside the auth_middleware + GovernorLayer stack,
inheriting rate limiting and JWT authentication automatically.

feat(migrations): add bundles, subscriber_bundles, airtime tables

00019_init_bundles.sql

• bundles table: bundle_id, name, bundle_type, individual allowance
  columns (data_bytes, voice_seconds, sms_count, roaming_data_bytes),
  validity_days, priority, amount_unconverted, is_active

00020_init_subscriber_bundles.sql

• subscriber_bundles table: FK to subscribers.imsi + FK to
  bundles.bundle_id, activated_at, expires_at, remaining allowance
  columns for deduction tracking

00021_init_airtime_balances.sql

• airtime_balances table: imsi, home_seconds, roaming_seconds,
  last_updated — durable source of truth synced from Redis

00022_init_airtime_transactions.sql

• airtime_transactions table: imsi, transaction_type (top_up/deduction/
  bundle_purchase/gift_sent/gift_received), seconds_delta, bundle_id,
  created_at — full audit trail

00023_alter_usage_records_charging_source.sql

• ALTER usage_records: adds charging_source TEXT ('bundle'|'credit')
  and subscriber_bundle_id INTEGER for billing reconciliation

Added airtime table migraiton