feat(media): add phased S3 layout migration

[bradseiler]

Why

Media payload keys are currently flat, so high-volume traffic lacks hash-leading S3 prefix entropy. Existing deployments also need a safe, explicit migration that preserves old reads and rollback behavior.

What

• moves media payloads to media/<sha[0:2]>/<sha[2:4]>/<community>/<filename> while leaving Git/CAS namespaces unchanged
• adds BUZZ_MEDIA_MIGRATION_PHASE with three complete read/write policies:
  • dual-read-legacy-write (default and upgrade-safe)
  • dual-read-dual-write
  • sharded-only
• adds new-first/legacy-fallback read telemetry, strict bucket classification, logical billing deduplication, duplicate-layout gauges, and authoritative payload keys in upload records
• includes buzz-media-layout-backfill and buzz-media-layout-delete-legacy in both relay image targets, with bounded paging, checkpoints, request-rate limiting, idempotency, destination verification, dry-run, and destructive confirmation
• documents new-install and upgrade flows and provides Kubernetes Job examples

Validation

• cargo test -p buzz-media --all-targets: 117 passed; live-MinIO test ignored
• cargo test -p buzz-relay --lib storage_sweep: 15 passed
• cargo clippy -p buzz-media -p buzz-relay --all-targets -- -D warnings: clean
• cargo fmt --all -- --check and git diff --check: clean
• both maintenance binary --help entry points built and ran locally
• pre-push hooks: Rust suites and desktop Tauri checks passed
• Docker image build could not run locally because the Docker daemon is unavailable; the Dockerfile explicitly builds, strips, and copies both binaries in debug and normal runtime targets

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