feat: Add benchmarks for image/audio vector storage and fix rand usage (#686)

- Implemented `criterion` benchmarks in `crates/synapse-infra/benches/storage_bench.rs` for:
    - Hologram encoding (~3ms)
    - Sled storage writes (~1.8ms)
    - Sled vector search (~2.4ms for 100 items)
- Updated `AesGcmAdapter` and `DiffusionAdapter` to use `rand` 0.9 syntax (`random()` instead of `gen()`, `rng()` instead of `thread_rng()`).
- Added `criterion` dependency to `crates/synapse-infra/Cargo.toml`.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: iberi22