[06] [M0.5] Persist respawns to a DB respawn table

[alseif0x]

Respawn queue is in-memory only -> respawns lost on restart.
C++: Map::SaveRespawnTime / respawn table. Rust target: wow-map spawn/respawn + DB.
Done: creature/GO respawn timers survive a server restart.

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Part of the PORT_PLAN. State: docs/migration/STATE.md · Defects: docs/migration/EXISTING-CODE-DEFECTS.md.
Done = runs live + byte/opcode-clean vs a C++ capture of the same action + exercised on a running server + C++ refs cited (STATE.md §5). 1 issue = 1 PR.