ECHO-930 feat(participant): prevent double start recording and track actual recording start time

[ussaama]

Double-start guards in the portal:

• ref latch on the initiate form (submit + skipOnboarding auto-submit) so a fast double click creates one conversation, not two; the auto-submit path now reuses startConversation and gains the missing conversation_started event
• re-entrancy latch and cancellation token in useChunkedAudioRecorder: a second tap during the mic prompt is a no-op, stopping during the prompt releases the acquired stream, and a failed start rolls back recorder, stream, and meter state so retries keep working
• recording_started analytics and the wake lock only fire when recording actually starts

recording_started_at:

• new nullable conversation.recording_started_at column (Directus snapshot)
• primary source: the first liveness ping with state "recording" persists the server-clocked start (same signal the live monitor already uses); the first audio chunk is the fallback, clamped to [created_at, now + 5m] and stored UTC
• one conditional PATCH enforces the rules in the filter itself: uploads are never stamped, the server-clocked liveness value may move the stamp earlier, and the client-clocked chunk value only fills an empty one
• text chunks and dashboard uploads never stamp; the conversations BFF retries without the field if the column is not deployed yet
• dashboard lists and the detail panel display recording_started_at with a created_at fallback; sorting stays on created_at (accepted trade-off)
• translations added for nl, de, fr, es; cs, it, uk fall back to English

[linear]

ECHO-930

[github-actions]

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