Add secp256k1 (ES256K) support to JWT access token verification using
Bluesky's indigo crypto package. This enables authentication from
external PDSes that use ES256K-signed tokens.

Changes:
- jwt.go: Add ES256K detection and verification using indigo's crypto
- New verifyES256KToken() for ES256K-specific verification
- New parseJWKMapToIndigoPublicKey() to convert JWK to indigo key
- New verifyJWTSignatureWithIndigoKey() for indigo signature verification
- New parseJWTClaimsManually() to parse claims without golang-jwt
- Update ToPublicKey() to return JWK map for secp256k1 curves

- did_key_fetcher.go: Return indigo PublicKey for secp256k1 keys
- FetchPublicKey now returns indigoCrypto.PublicKey for secp256k1
- NIST curves (P-256, P-384, P-521) still return *ecdsa.PublicKey

This complements the DPoP ES256K support added earlier, completing
full ES256K support across the authentication stack.

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

- Add ES256K (secp256k1) algorithm support using indigo's crypto package
- Add algorithm-curve binding validation to prevent algorithm confusion attacks
- Restore exp/nbf claim validation for DPoP proofs (security regression fix)
- Replace golang-jwt parsing with manual JWT parsing to support ES256K
- Add comprehensive test coverage for ES256K and security validations
- Update Caddyfile with proper Host headers for DPoP htu matching

Security fixes:
- Validate JWK curve matches claimed algorithm (ES256K->secp256k1, ES256->P-256, etc.)
- Validate exp claim if present (with clock skew tolerance)
- Validate nbf claim if present (with clock skew tolerance)

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

DPoP security improvements addressing PR review findings:

- feat(auth): comprehensive DPoP security improvements
- Access token hash (ath) validation per RFC 9449
- Proxy header support (X-Forwarded-Host, RFC 7239 Forwarded)
- EscapedPath for percent-encoded URLs
- Case-insensitive DPoP scheme per RFC 7235

- fix(auth): prevent goroutine leak from DPoP replay cache
- Graceful server shutdown with signal handling
- Proper cleanup in integration tests

- docs: update authentication documentation for DPoP scheme

Add comment clarifying that PDS uploadBlob calls use Bearer scheme
(standard atproto server auth) rather than DPoP (AppView client auth).

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

Update documentation to reflect the transition from Bearer tokens
to DPoP-bound tokens for client authentication:

- federation-prd.md: Update auth examples to use DPoP scheme
- Add note about Bearer vs DPoP for server-to-server auth
- Update request examples with DPoP header

- COMMENT_SYSTEM_IMPLEMENTATION.md: Update auth references
- "Bearer token" → "DPoP-bound access token"
- Document DPoP proof validation in OptionalAuth

- FEED_SYSTEM_IMPLEMENTATION.md: Update curl examples
- Add DPoP header alongside Authorization header
- Update auth requirement description

- PRD_OAUTH.md, aggregators/SETUP_GUIDE.md, auth/README.md:
- Minor terminology updates for consistency

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

The DPoP verifier starts a background goroutine for nonce cache cleanup.
Without calling Stop(), this goroutine persists and accumulates across
server reloads and test runs.

Changes:
- cmd/server/main.go: Add graceful shutdown with signal handling
- Listen for SIGINT/SIGTERM
- Call authMiddleware.Stop() during shutdown
- Use http.Server.Shutdown() for graceful connection draining

- Integration tests: Add defer authMiddleware.Stop() after creation
- user_journey_e2e_test.go
- post_e2e_test.go
- community_e2e_test.go
- aggregator_e2e_test.go
- jwt_verification_test.go (2 locations)

This prevents NonceCache cleanup goroutines from leaking in both
production and test environments.

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

This commit addresses multiple security findings from PR review:

1. Access Token Hash (ath) Validation (RFC 9449 Section 4.2)
- Added VerifyAccessTokenHash() to verify DPoP proof's ath claim
- If ath is present, it MUST match SHA-256 hash of access token
- Prevents proof reuse across different tokens

2. Proxy Header Support for htu Verification
- Added extractSchemeAndHost() for X-Forwarded-Proto/Host support
- RFC 7239 Forwarded header parsing with mixed-case keys and quotes
- Critical for DPoP verification behind TLS-terminating proxies

3. Percent-Encoded Path Handling
- Use r.URL.EscapedPath() instead of r.URL.Path
- Preserves percent-encoding for accurate htu matching

4. Case-Insensitive DPoP Scheme (RFC 7235)
- Added extractDPoPToken() helper with strings.EqualFold()
- Accepts "DPoP", "dpop", "DPOP" per HTTP auth spec

Tests added for all security improvements:
- TestVerifyDPoPBinding_UsesForwardedHost
- TestVerifyDPoPBinding_UsesStandardForwardedHeader
- TestVerifyDPoPBinding_ForwardedMixedCaseAndQuotes
- TestVerifyDPoPBinding_AthValidation
- TestRequireAuth_CaseInsensitiveScheme

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

The latest indigo (Nov 27, 2025) requires Go 1.25 which isn't available
in Docker Hub yet. Pin to the Oct 10, 2025 commit which is the last
Go 1.24-compatible version.

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

The local Go 1.25.1 is a pre-release version not available in Docker Hub.
Set minimum go version to 1.24 with toolchain directive for local dev.

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

Implement DPoP (RFC 9449) token binding for OAuth access tokens.

Features:
- DPoP proof verification with ES256 signing
- NonceCache for jti-based replay protection
- JWK thumbprint calculation per RFC 7638
- Middleware integration with Stop() for clean shutdown
- X-Forwarded-Proto support for reverse proxy deployments

Security:
- DPoP is additional security, never a fallback
- Tokens with cnf.jkt require valid DPoP proof
- 5-minute proof validity window
- Replay attack prevention via jti tracking

- Add github.com/google/uuid for DPoP proof jti generation
- Add .cache/ to .gitignore for Go build cache

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