refactor: drop dead MQTT/FTPS leftovers and unify pin + redaction

[DLANSAMA]

AAA stack 1/9. Base: main (includes #108).

Dead-code and unification pass. No behaviour change for a working printer.

• Drop unused MQTT PEM / _TRUSTED_CERT_FILE leftovers
• Delete the unused FTPS connection pool; get_ftp_client closes instead of quit
• Single fingerprint normalizer: tlspin.normalize_fingerprint
• emit_json uses jsonio.redact_url_credentials instead of a second copy
• Fix the tautological assert_shape helper

Stack (merge in order):

1. refactor: drop dead MQTT/FTPS leftovers and unify pin + redaction #110 this
2. fix: make the camera streamer opt-in and move snapshot out of protocols #111 snapshot / streamer opt-in
3. refactor: split MQTT into tls/cmd/print/monitor and pin via SSLContext #112 MQTT split + PinningSSLContext
4. refactor: emit --json through contracts; stop flattening status #113 contracts / unflatten status
5. fix: reuse one MQTT TLS session for TUI dashboard and monitor #114 TUI MQTT session reuse
6. fix: run the TUI on Textual 8 and drop Python 3.9 #115 Textual 8 + drop 3.9
7. test: stop stubbing paho in sys.modules #116 stop stubbing paho
8. refactor: commands raise; cli.main is the sole error emitter #117 sole error path
9. fix: size_verified, leftover contracts, and extras on raised errors #118 size_verified + leftover contracts

Local gates on the full stack: ruff, mypy, bandit, layers, schemas, pytest -W error::ResourceWarning -m "not live" — 1436 passed, 88.91% coverage.