Fix Storacha client principal assumption to support all DID types

[sujalsalekar]

Fix Storacha client principal assumption to support all DID types

Description

Issue Summary

The Storacha client currently enforces a did:key requirement for principals, which is unnecessarily restrictive and could cause compatibility issues with other DID types.

Current Implementation

// app/lib/server/jobs.ts:237-245
const agentData = new AgentData({
  // FIXME: The Storacha client thinks a principal has to be a `did:key`,
  // which is a bit silly. All DIDs have keys, and any `Signer` by
  // definition has its private key loaded and can sign.
  principal: getServerIdentity() as unknown as Signer<DID<'key'>>,
  // ... rest of configuration
})

Problem

• The code forces a type cast to Signer<DID<'key'>> even though the actual identity may be a different DID type
• This assumption is incorrect - all DIDs have keys, and any Signer can sign regardless of DID type
• Could cause TypeScript errors or runtime issues if other DID types are used
• Limits flexibility for future DID implementations

Impact

• Severity: Medium
• Risk: Future breakage or security issues if DIDs other than did:key are used
• Affected Areas: Storacha client initialization, authentication, signing operations