feat: migrate to Veilid 0.4.8 and fix test infrastructure

Migrates save-rust to Veilid 0.4.8 API and fixes test infrastructure.

## Veilid 0.4.8 API Changes
- Updated src/utils.rs: CryptoKey → RecordKey, TypedKey → CryptoTyped
- Updated src/server.rs: Removed vld0_generate_keypair import (now private)
- Public API unchanged: Android bindings remain fully compatible

## Test Infrastructure Fix
- Root cause: All tests shared global BACKEND static with same Veilid namespace
- Solution: Each test gets unique namespace (test_name + timestamp)
- Added conditional compilation for BACKEND static:
  - Production: OnceCell (efficient, set-once)
  - Tests: RwLock (resettable between tests)
- Fixed double initialization: Removed backend.start() after from_dependencies()
- Proper cleanup: Added set_backend()/clear_backend() test helpers

## Code Quality
- Fixed clippy warnings (inline format args in logging.rs, media.rs)
- Tests: 7+/10 passing (was 0/10)
- No more "Veilid already initialized" errors

## Dependencies
- save-dweb-backend: v0.2.0
- veilid-core: v0.4.8
- Version bumped to 0.2.0

Author: tripledoublev