Handle Bluesky profile updates gracefully

[disnet]

Summary

• Frontend profile refresh: On app init and tab visibility change, fetch fresh profile data from Bluesky public API (non-blocking, uses existing ProfileService with 5-min TTL cache). Updates avatar, handle, and display name if changed.
• Avatar error fallback: Broken avatar URLs now show a placeholder circle instead of a broken image icon, in both the sidebar and settings page.
• Backend stale profile refresh: handleAuthMe checks profile_updated_at and re-fetches from Bluesky if >24h stale, updating both the session and users table.
• Migration: Adds profile_updated_at column to users table.

Test plan

• Change Bluesky profile picture, reload Skyreader — avatar updates within seconds
• Return to Skyreader tab after changing profile — avatar updates on focus
• Simulate broken avatar URL — placeholder shown instead of broken image
• Handle/display name changes also picked up
• Backend tests pass (88/88)
• No excessive API calls (ProfileService TTL prevents spam)

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