Bound startup retry loop and add SKIP_STARTUP_CHECKS env flag

[Copilot]

run_startup_checks() contained an unbounded while retry loop — if Hawki was unreachable or ALLOWED_KEYS was misconfigured, the app would hang indefinitely, blocking TestClient-based tests and production startup alike.

Changes

• SKIP_STARTUP_CHECKS (default: false): Bypasses all startup checks when true. Background cleanup task is still scheduled. Intended for test/dev environments.
• STARTUP_MAX_RETRIES (default: 0 = unlimited): Caps the Hawki endpoint retry loop. On exhaustion, logs an error and returns — service continues in degraded state rather than blocking forever.
• Invalid STARTUP_MAX_RETRIES value raises a ValueError with a descriptive message at startup.

# .env or environment
SKIP_STARTUP_CHECKS=true        # skip entirely in tests
STARTUP_MAX_RETRIES=5           # give up after 5 attempts in prod

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