[curtishall]

Batch of issue fixes:

• The events duration do not represent their actual duration #296: events use the server-provided media_duration when available, falling back to updated - published when it is missing or zero (the server reports 0 for unknown). Adds Event.mediaDuration plus parsing in the JSON feed branch.
• Stream don't always load automatically when switching views on iOS mobile #341: switching mobile tabs created players progressively without notifying, so tiles built early stayed empty until a manual reload. initializeDevices now notifies as each player is created.
• Timeline scrubbing re-opens video on every movement with no debounce #363: timeline scrubbing re-opened every tile player on each movement. Seeks now update the position immediately but wait 350ms for scrubbing to settle before re-opening; overlapping setDataSource calls are serialized; re-opens no longer accumulate duplicate player subscriptions.
• Streams not playing at all #345 (diagnostic only, not closed): desktop fps stat returned hardcoded 0.0 due to an inverted guard and threw on Pi. Now reports the real codec frame rate on desktop.

Verification: flutter analyze clean, full suite 36/36 green including new event_duration_test.dart and debouncer_test.dart.

Closes #296
Closes #341
Closes #363