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fix: harden the subscription path (non-fatal snapshot, no-emitter backoff, drop metric) (#424)

Server-side robustness for the subscription streams, complementing the client-side
re-subscribe fixes:

- subscribe-to-friend-connectivity: make the initial online-friends snapshot best-effort.
  A DB/registry hiccup no longer tears down the whole subscription (which forced the client
  to reconnect and re-run the queries, churning); log a warning and continue to live updates.
  (peersStats.getConnectedPeers already swallows its own errors, so the real risk was
  getOnlineFriends / getProfiles.)
- handleSubscriptionUpdates: on the "no emitter" path (RPC call live but the per-address
  emitter is gone — an abnormal/racy state), throw instead of returning. A clean completion
  invited an instant re-open into a hot loop; erroring makes a well-behaved client back off.
- emitterToGenerator: add an onOverflowDrop hook + subscription_updates_dropped_total metric
  so silent value-queue drops (a slow consumer missing updates) are observable.
- Keep the per-occurrence "Duplicate subscription detected" debug log alongside the
  subscription_duplicates_total metric, for per-connection visibility into re-subscribe churn.

Tests cover: non-fatal snapshot (continues), live-update stream error (logs + propagates),
no-emitter throw, duplicate (metric + debug log), and the value-queue drop hook.
Build, lint and full unit suite (106 suites / 1616 tests) green.

2296 of 2685 branches covered (85.51%)

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5 of 6 new or added lines in 3 files covered. (83.33%)

5758 of 6221 relevant lines covered (92.56%)

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