refactor: update dependencies and remove unused code

- Added new dependencies: `adler2`, `crc32fast`, `flate2`, `miniz_oxide`, and `libredox`.
- Updated existing dependencies: `tokio-rustls` to version 0.26.4 and `filetime` to version 0.2.27.
- Removed the `backup.rs` file as it is no longer needed.
- Introduced tests for configuration and credential management.
- Enhanced the `identity` module to generate W3C compliant DID documents.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/adr/002-did-key-method.md

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+# ADR-002: DID Key Method for Node Identity
+
+**Status**: Accepted
+**Date**: 2026-03
+
+## Context
+
+Each Archipelago node needs a cryptographic identity for peer authentication, federation, and verifiable credentials. Multiple DID methods exist (did:web, did:ion, did:key, did:peer).
+
+## Decision
+
+Use `did:key` as the primary DID method.
+
+## Consequences
+
+### Positive
+- **Self-contained**: The DID document is derived entirely from the public key — no external resolution needed
+- **Offline-capable**: Works without internet, aligning with sovereignty principles
+- **Simple**: No registration, no blockchain, no web server required
+- **Fast**: DID resolution is a local computation, not a network request
+- **Ed25519**: Uses Ed25519 keys which are fast, compact, and well-supported
+
+### Negative
+- **No key rotation**: The DID is bound to a single key; rotating requires a new DID
+- **No service endpoints in DID**: Cannot embed service URLs in the DID document itself
+- **No revocation**: Cannot revoke a did:key without out-of-band mechanisms
+
+### Mitigation
+- Use federation trust lists for key management and revocation
+- Store service endpoints (onion address, pubkey) separately in federation state
+- Support migration to did:peer or did:web in future versions if key rotation is needed