Add new simplified authentication system:
- JWT parsing and validation against atProto standards
- JWKS fetcher with caching for PDS public keys
- Support for both signature verification and parse-only modes
- Claims extraction (sub, iss, aud, exp, iat)

Dependencies:
- Add github.com/golang-jwt/jwt/v5 for JWT handling

This replaces the complex OAuth/DPoP flow with direct JWT validation,
suitable for alpha phase where we control both the PDS and AppView.

Files:
- internal/atproto/auth/jwt.go: JWT parsing and verification
- internal/atproto/auth/jwks_fetcher.go: Public key fetching
- internal/atproto/auth/jwt_test.go: Test coverage
- internal/atproto/auth/README.md: Documentation

Remove old OAuth/DPoP implementation that was replaced with simpler
JWT-based authentication:
- Removed OAuth handlers (login, callback, logout, metadata, JWKS)
- Removed DPoP proof generation and transport layer
- Removed OAuth client with PAR/PKCE flows
- Removed OAuth session management and repository
- Removed OAuth integration tests

This implementation was too complex for alpha phase and has been
replaced with direct JWT validation against PDS JWKS endpoints.

See docs/PRD_OAUTH.md for the new simplified approach.

Major updates to community PRDs based on lexicon vs implementation
gap analysis:

PRD_COMMUNITIES.md:
- Add "Alpha Blockers" section with clear must-haves
- Mark 6 XRPC endpoints as E2E tested (create, get, list, update,
subscribe, unsubscribe)
- Reorganize into Alpha vs Beta priorities
- Defer posts, wiki, moderation, membership to Beta
- Add note to remove unused categories field

PRD_BACKLOG.md:
- Add P1: Subscription visibility level (1-5 scale feed slider)
- Add P1: Community blocking implementation
- Add P2: Remove categories cleanup task
- Mark OAuth and token refresh as ALPHA BLOCKERS

PRD_GOVERNANCE.md:
- Update status: basic authorization exists (creator-only updates)
- Clarify moderator management is post-alpha (Beta Phase 1)

Key Findings from Analysis:
- Lexicon has many features not yet implemented (wiki, blocking,
advanced rules)
- Current alpha scope: basic CRUD + OAuth + token refresh
- Categories field exists but adds no value - marked for removal
- Membership design decisions deferred to Beta

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Adds comprehensive end-to-end tests for community XRPC endpoints:

Update Endpoint:
- Tests full write-forward flow (HTTP → PDS → Firehose → AppView)
- Verifies displayName, description, visibility updates
- Confirms CID changes after update
- Validates AppView indexing via Jetstream consumer

Subscribe/Unsubscribe Endpoints:
- Tests subscription creation in user's repository
- Verifies records written to PDS and queryable
- Tests unsubscribe deletes records from PDS
- Validates write-forward pattern for user actions

All tests use real PDS instance and Jetstream firehose for
true end-to-end validation.

Test Results: All 6 core XRPC endpoints now have E2E coverage
- create, get, list, update, subscribe, unsubscribe

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Fixes data loss bug where moderationType and contentWarnings
would be erased if not explicitly provided in update request.

Previously, omitting these fields in an update request would
remove them from the PDS record. Now follows the same pattern
as other optional fields (displayName, description, etc.) by
preserving existing values when not being updated.

Impact: Prevents accidental erasure of moderation configuration
when updating other community properties.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Implements social.coves.community.update endpoint for updating
community profiles (displayName, description, visibility, etc.).

Changes:
- Add UpdateHandler with XRPC endpoint support
- Register update route in community routes
- Uses community's own PDS credentials (V2 architecture)
- Preserves OAuth TODO for production deployment

Related: Update endpoint was defined in service layer but had
no HTTP handler or route registration.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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