[Improve] Deliver user notifications consistently across chat providers

[roomote-roomote]

​Created by Roomote. View the task or mention @roomote-roomote for follow-up asks.

What changed

• Replaced separate identity, single-provider send, and best-effort send APIs with one typed delivery attempt that reports unlinked, sent, or failed.
• Made platform issue alerts derive admin eligibility and first-success delivery from the attempt result without a duplicate identity lookup.
• Made GitHub installation approval notifications attempt every supported communication provider, including Discord.

Why this change was made

Identity preflights repeated provider mapping and destination work before delivery, while overlapping wrappers had already drifted to different provider coverage. A single provider-owned attempt contract keeps destination resolution and delivery outcomes aligned as communication providers evolve.

Impact

Platform issue retries retain their existing all-eligible-admin completion semantics with fewer database lookups. Installation approval notifications can now reach Discord-linked users, and callers can distinguish an unlinked user from an attempted delivery failure.

[roomote-community]

No code issues found. See task

_{Reviewed 1055e3f}