feat: configurable chunk size via TOML config (default 5 MB)

[rubenhensen]

Summary

• Remove the hardcoded CHUNK_SIZE constant (was 10 MiB) from main.rs
• Add an optional chunk_size field to CryptifyConfig in config.rs, defaulting to 5,000,000 bytes (5 MB) when not specified in TOML
• Update upload_chunk validation and Rocket body-size limits to use the configured value
• Add chunk_size = 5000000 example to conf/config.dev.toml

Test plan

• Verify cargo check passes (done locally)
• Deploy with no chunk_size in TOML and confirm the default 5 MB limit applies
• Deploy with a custom chunk_size value and confirm it is respected
• Upload a chunk exceeding the configured limit and verify the error message shows the correct byte count