Implement PostgreSQL repository for community blocking with:
- BlockCommunity: Create/update block (idempotent via ON CONFLICT DO UPDATE)
- UnblockCommunity: Remove block
- GetBlock: Retrieve block by user/community DIDs
- GetBlockByURI: Retrieve block by AT-URI (for Jetstream DELETE ops)
- ListBlockedCommunities: Paginated list of user's blocks
- IsBlocked: Fast boolean check using EXISTS

All methods use direct string values (not sql.NullString) since
record_uri and record_cid are NOT NULL in the schema.

Fixes: PR review comment #2, #3 (P0 - Critical)

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

Create migration for community blocking feature with:
- user_did and community_did with widened regex validation
- blocked_at timestamp for chronological ordering
- record_uri and record_cid for atProto federation
- Unique constraint on (user_did, community_did) pairs
- Indexes for efficient queries

DID regex pattern ^did:(plc|web):[a-zA-Z0-9._:%-]+$ supports:
- Uppercase and lowercase letters
- Dots for did:web domains (e.g., did:web:coves.social)
- Hyphens, underscores, colons, percent signs per DID spec
- Anchored to end for strict matching

Fixes: PR review comment #1 (P0 - Critical)

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

Define lexicon for community blocking records following atProto
conventions similar to app.bsky.graph.block.

Block records:
- Live in user's repository (at://user_did/social.coves.community.block/{tid})
- Are public (blocks are not private)
- Contain subject (community DID) and createdAt timestamp
- Use TID-based keys for chronological ordering

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

Add utility functions for working with atProto records:
- ExtractRKeyFromURI: Extract record key from AT-URI
- StringFromNull: Convert sql.NullString to string
- ParseCreatedAt: Parse createdAt timestamps from records

ParseCreatedAt preserves chronological ordering during Jetstream
replays and backfills by extracting the original record timestamp
instead of using time.Now().

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

Complete implementation of subscription indexing with contentVisibility slider.

This resolves a critical alpha blocker by enabling:
✅ Real-time subscription indexing from Jetstream firehose
✅ ContentVisibility (1-5 feed slider) for user customization
✅ Atomic subscriber count management
✅ Feed generation infrastructure (ready for next phase)

Summary of changes:
- New lexicon: social.coves.community.subscription
- Migration 008: content_visibility column with constraints
- Production Jetstream consumer running in cmd/server/main.go
- Full implementation across handler → service → consumer → repository
- 13 comprehensive integration tests (all passing)
- Enhanced E2E tests verifying complete flow
- Fixed critical collection name bug (unsubscribe now works)

atProto Compliance:
- Singular namespace (community not communities)
- Standard field naming (subject not community)
- Follows Bluesky graph record conventions

Testing: All 32 integration tests passing ✅

Closes alpha blocker: Subscription indexing & feed slider

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

Updated PRD_COMMUNITIES.md:
- Moved subscription indexing from "Alpha Blockers" to "Completed" section
- Documented all 8 fixes implemented (collection name bug, contentVisibility,
production consumer, migration, atomic counts, DELETE ops, idempotency,
atProto compliance)
- Updated impact section: AppView indexing ✅, feed generation enabled ✅,
accurate subscriber counts ✅
- Added testing coverage: 13 integration tests, enhanced E2E tests
- Updated "Last Updated" date to 2025-10-16
- Added file references for all implementation components

Updated PRD_BACKLOG.md:
- Marked "Subscription Visibility Level (Feed Slider)" as ✅ COMPLETE
- Changed status from "P1: ALPHA BLOCKER" to "✅ DONE"
- Added completion date: 2025-10-16
- Documented complete solution with all files modified
- Listed impact: Users can now adjust feed volume per community

Impact on Alpha Launch:
- ✅ Critical alpha blocker resolved
- ✅ Feed generation infrastructure ready
- ✅ ContentVisibility (1-5 scale) fully implemented
- Remaining blocker: Community blocking feature

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