perf(merope/sabnzbd): negotiate ChaCha20 instead of AES

The Cortex-A72 in the BCM2711 implements no ARMv8 crypto extensions, so
TLS runs entirely in software. Measured on merope: AES-256-GCM manages
53 MB/s per core against 208 MB/s for ChaCha20-Poly1305, roughly 4x.

All six providers default to TLS_AES_256_GCM_SHA384 and all six accept
ChaCha20, so at the 43 MB/s this box was pulling, about 0.8 of a core was
going to decryption alone -- on a machine already sitting at 93% CPU with
the network only a third utilised and the disks coasting. Usenet
throughput here is bounded by the cipher, not by bandwidth or IO.

Setting ssl_ciphers makes SABnzbd cap the connection at TLS 1.2, since
Python never exposed SSL_CTX_set_ciphersuites() for the 1.3 suites. That
costs an extra handshake round trip, amortised across long-lived
connections, and an unencrypted certificate that SNI already gives away.
ECDHE preserves forward secrecy and ChaCha20-Poly1305 is the same AEAD
1.3 would have picked.

Preferred over exporting OPENSSL_CONF, which reaches the same cipher
while keeping TLS 1.3: this is the supported knob, so when upstream can
select 1.3 ciphersuites the cap lifts on its own, whereas overriding a
library's config file to reorder one list is the kind of thing that
breaks silently.

Assisted-by: pi (claude-opus-5)

hosts/nixos/merope/services/media/sabnzbd.nix

@@ -22,12 +22,24 @@
       [[music]]
       name = music
 
+      # This Pi's Cortex-A72 has no ARMv8 crypto extensions, so TLS is decrypted
+      # in software: AES-256-GCM manages 53 MB/s per core against 208 MB/s for
+      # ChaCha20-Poly1305. Every provider defaults to AES and all six accept
+      # ChaCha20, so asking for it moves most of a core off decryption.
+      #
+      # Setting ssl_ciphers makes SABnzbd cap the connection at TLS 1.2, because
+      # Python does not expose SSL_CTX_set_ciphersuites() for the 1.3 suites.
+      # ECDHE keeps forward secrecy and ChaCha20-Poly1305 is the same AEAD that
+      # 1.3 would use, so the downgrade costs an extra handshake round trip and
+      # an unencrypted certificate, nothing more. Drop these lines once upstream
+      # can select 1.3 ciphersuites.
       [servers]
       [[frugal]]
       enable = 1
       name = frugal
       host = sanews.frugalusenet.com
       ssl = 1
+      ssl_ciphers = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
       port = 563
       username = misterio
       password = ${config.sops.placeholder.frugalusenet-key}
@@ -38,6 +50,7 @@
       name = frugal-secondary
       host = news.frugalusenet.com
       ssl = 1
+      ssl_ciphers = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
       port = 563
       username = misterio
       password = ${config.sops.placeholder.frugalusenet-key}
@@ -48,6 +61,7 @@
       name = frugal-bonus
       host = bonus.frugalusenet.com
       ssl = 1
+      ssl_ciphers = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
       port = 563
       username = misterio
       password = ${config.sops.placeholder.frugalusenet-key}
@@ -58,6 +72,7 @@
       name = eweka
       host = news.eweka.nl
       ssl = 1
+      ssl_ciphers = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
       port = 563
       username = 043b11d25e1d9f6f
       password = ${config.sops.placeholder.eweka-key}
@@ -68,6 +83,7 @@
       name = blocknews
       host = sanews.blocknews.net
       ssl = 1
+      ssl_ciphers = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
       port = 563
       username = misterio
       password = ${config.sops.placeholder.blocknews-key}
@@ -78,6 +94,7 @@
       name = blocknews-secondary
       host = usnews.blocknews.net
       ssl = 1
+      ssl_ciphers = ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305
       port = 563
       username = misterio
       password = ${config.sops.placeholder.blocknews-key}