Restore intentional fail-fast auth ordering; harden keyId and config validation

[Copilot]

The previous review reordered auth middleware to verify the Ed25519 signature before zone/method authorization checks. This was intentional design: cheap authorization lookups run first to reject unauthorized requests without paying cryptographic cost.

Middleware ordering (restored)

Both zoneAuthMiddleware and dataAuthMiddleware now follow the original fail-fast sequence:

1. Parse header → timestamp check → key lookup
2. Authorization checks (zone membership, method flags) → 403 fast on denial
3. Signature verification (Ed25519) → 401 only for authenticated-but-invalid requests

The previous session had inverted steps 2 and 3.

Retained from previous review

• Normalized path in canonical string — zoneAuthMiddleware now verifies the signature against /zones/<lowercase-zone> rather than the raw r.URL.Path, eliminating a case-sensitivity inconsistency between authorization and verification.
• keyId character validation — ParseAuthHeader rejects keyIds containing /, \, spaces, or other characters unsafe in map keys and filesystem paths (^[a-zA-Z0-9._-]+$).
• Config format validation — server.format and per-key format must be "", "yaml", or "raw"; invalid values now cause a startup error.

Tests

Replaced the three "sig-before-authz" tests with "authz-before-sig" equivalents that assert 403 (not 401) is returned when zone/method authorization fails, even with a bad signature. Renamed TestZoneAuth_InvalidKeyIDChars → TestAuth_InvalidKeyIDChars since keyId parsing applies to both middlewares.

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