feat: observability metrics and live vote tallies (#382, #364)

[testersweb0-bug]

Summary

Closes #382 and #364

• Observability ([Observability] Add structured logging + metrics for RPC and transaction failures #382): Emit structured events for every contract error code, simulation failure, submission failure, confirmation timeout, and RPC error. Events are logged as JSON, posted to /api/observability/events for aggregation, and optionally forwarded to an external webhook. The admin observability dashboard visualizes rolling failure rates and surfaces alerts when thresholds are exceeded.
• Live vote tallies ([Feature] Live vote tallies via campaign_vote_cast events #364): Poll Soroban campaign_vote_cast events on campaign detail and increment approve/reject counts without refetching the full campaign. Quorum and approval-threshold progress in VotingComponent recompute from live props. Falls back to on-chain vote getter polling when event streaming is unavailable.

Changes

Area	Files
Observability core	src/lib/observability/*
Contract instrumentation	src/lib/contractClient.ts
API	src/app/api/observability/events/route.ts, metrics/route.ts
Admin dashboard	src/components/admin/ObservabilityDashboard.tsx, src/app/[locale]/admin/observability/page.tsx
Live votes	src/lib/sorobanEvents.ts, src/hooks/useCampaignVoteEvents.ts, src/hooks/useLiveVoteTallies.ts
Detail page	src/app/[locale]/causes/[id]/CauseDetailClient.tsx
Config	.env.example
Tests	observability.test.ts, sorobanEvents.test.ts, useLiveVoteTallies.test.tsx

How it works

Observability

1. Any Soroban RPC or transaction failure in contractClient calls recordObservabilityFailure / recordObservabilityKind.
2. Successful transactions emit transaction_success for rate denominators.
3. The browser POSTs events to /api/observability/events; the server keeps a rolling buffer and computes 5-minute rates.
4. GET /api/observability/metrics returns counters, rates, active alerts, and recent events.
5. Admins open Admin → RPC & transaction metrics (/admin/observability) for the dashboard.

Live votes

1. useLiveVoteTallies loads initial approve/reject counts from get_approve_votes / get_reject_votes.
2. useCampaignVoteEvents polls getEvents for ("campaign_vote_cast", campaign_id, *) every 5s.
3. New events increment upvotes or downvotes from the approve: bool payload.
4. If mocks are enabled or the contract is unset, polling reconciliation runs every 30s instead.

Configuration

NEXT_PUBLIC_USE_MOCKS=false
NEXT_PUBLIC_CONTRACT_ADDRESS=C...

# Live votes
NEXT_PUBLIC_VOTE_EVENTS_POLL_MS=5000
NEXT_PUBLIC_VOTE_TALLIES_POLL_MS=30000

# Observability
NEXT_PUBLIC_OBSERVABILITY_WEBHOOK_URL=          # optional external sink
OBSERVABILITY_ALERT_SIMULATION_FAILURE_RATE=0.15
OBSERVABILITY_ALERT_SUBMISSION_FAILURE_RATE=0.1
OBSERVABILITY_ALERT_RPC_TIMEOUT_RATE=0.2

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