Fix tempo-config for Tempo 3 (otel-lgtm:0.29.0)

[grey275]

Problem

Found while running a from-scratch docs/INSTALLING.md install to vet the Grafana stack: traces never reach Grafana on a clean deploy.

The pinned grafana/otel-lgtm:0.29.0 ships Tempo 3.0.2, which removed:

• the compactor component (#6273)
• the metrics-generator local-blocks processor (#6555)

But the committed observability/tempo-config.yaml still carried the Tempo-2.x forms of both deltas: a top-level compactor: block, metrics_generator.processor.local_blocks, metrics_generator.traces_storage, and a local-blocks entry in the overrides processors list. Tempo fails config parse:

line 39: field compactor not found in type app.Config
line 46: field local_blocks not found in type generator.ProcessorConfig
line 53: field traces_storage not found in type generator.Config

and exits silently (logging=false). The container stays healthy, the other five components report ready, logs and metrics keep flowing — the only symptom is traces going dark. Exactly the failure mode observability/README.md warns about. (PR #37 fixed the overrides format for Tempo 3 but not these three fields.)

Fix

Re-author both deltas for Tempo 3:

• 72h block_retention now lives on backend_scheduler.provider.compaction.compaction and backend_worker.compaction (the scheduler/worker that replaced the compactor). The nested provider.compaction.compaction path is confirmed against Tempo's own /status/config — the -help flag paths are mangled for this struct.
• Drop the local-blocks processor and traces_storage. TraceQL metrics ({...} | rate() by(...), which the Traces Drilldown issues) are served natively in Tempo 3 without the processor.

Also updates the README mount-contract row to match.

Verification

• tempo -config.verify=true on the committed file: parses clean
• /status/config after startup: block_retention: 72h0m0s on both scheduler and worker
• Applied to a live install: Tempo comes up ready, traces are searchable, /api/metrics/query_range with {} | rate() returns series
• Metrics (Prometheus slm_*) and logs (Loki) unaffected

Not a frontend change; no dev-server link. Reviewers can see it running in the vetting install at http://localhost:3001 (Grafana admin/admin).

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