Retire or clearly label legacy faucet.py (records drips, sends nothing) + fix FAUCET.md + audit mock_mode

[Scottcjn]

Fallout from #8240 (the one confirmed finding).

• Root faucet.py drip() writes drip_requests and returns {ok:true, amount:0.5} with UI copy 'Sent X RTC', but contains no node call at all (faucet.py:383-432). Anyone running it per FAUCET.md gets a faucet that lies to every caller.
• The real faucet is faucet_service/faucet_service.py (POST /wallet/transfer with X-Admin-Key, records tx_hash, wired into testnet/systemd/rustchain-testnet-faucet.service). But its mock_mode defaults to True (faucet_service.py:806), so even the real one is a no-op unless explicitly configured.

Tasks:

• Retire root faucet.py, or stamp it '# DEMO - records requests, does not pay' and change the success copy so it does not claim RTC was sent
• Update FAUCET.md to point at faucet_service/ as the only supported faucet
• Flip mock_mode default to False, or fail loudly at startup when mock_mode is on
• Audit deployed faucet_config.yaml: mock_mode false, RC_ADMIN_KEY present

Credit: surfaced in #8240 Additional Observation (aiidentificationmachines-coder), verified by Vyacheslav-Tomashevskiy and two independent adjudication agents.

[Vyacheslav-Tomashevskiy]

/claim #8243