feat(observability): centralized admin-tunable event log levels

[emmyoat]

Closes #95

Summary

Introduces a centralized logging abstraction so mainnet builds get runtime
log-level control instead of the old compile-time-only
#[cfg(feature = "logging")] flag.

• New src/observability.rs: LogLevel { Debug, Info, Warn, Error } enum
  and log(env, level, topics, data) — the single choke point every
  structured event now routes through.
• Per-network compiled default (Debug on dev, Info on testnet
  feature, Warn on mainnet feature), overridable at runtime.
• set_log_level(env, caller, level) — admin-only (require_auth +
  require_admin), durably persisted so it survives calls/upgrades.
  get_log_level(env) added as a companion view.
• Every helper in events.rs rewritten to call log() with an assigned
  severity. Event names and payloads are unchanged (additive only).
• The three ad-hoc #[cfg(feature = "logging")] blocks in lib.rs
  (swap, safe_swap) now go through the same runtime-gated log()
  call instead of a compile-time-only flag.
• README "Observability" section documents levels, per-network defaults,
  the admin override flow, and guidance for adding new events.

Test plan

• observability_tests.rs covers:
  • set_log_level rejects non-admin callers
  • level persists durably across calls
  • Debug threshold lets all 4 severities through
  • Warn threshold allows only Warn/Error (event-count assertion)
  • Error threshold silences everything except Error-level events
    (event-count assertion, per acceptance criteria)
• cargo test --workspace --lib

[ameeribro4-sudo]

Great job consolidating event log levels behind a single admin-tunable knob — much friendlier to operators. CI is green. Clean implementation, merging 🚀

[ameeribro4-sudo]

The centralized log-level knob is a clean follow-up — thanks for the work 👋

Heads up: while processing the queue, I merged PRs #101, #102, #103, #104-attempt, #105-attempt, and #110 ahead of yours. This branch edited lib.rs, events.rs, and storage.rs — and so did several of those — so it's now behind main with merge conflicts.

Could you rebase onto the latest main and push? After CI is green on the rebased tip I'll merge it. Ping me if you want a hand.