obsidian: self-hosted vault sync via CouchDB on jupiter

Adds services/dev/obsidian-livesync.nix — CouchDB 3 from the native
nixpkgs module, tuned as the backend for the Self-hosted LiveSync plugin
— and publishes it as notes.mgaction.town through neptun.

It goes out over the public reverse proxy rather than staying on the LAN
because Obsidian's mobile apps refuse cleartext HTTP and *.jupiter.sol
cannot hold a publicly trusted cert. That makes the hardening load-bearing
rather than decorative:

  - require_valid_user in both [chttpd] and [chttpd_auth], so nothing
    answers unauthenticated on the open internet;
  - neptun's vhost matches on CouchDB's own naming rule (system endpoints
    all begin with `_`, user databases never can), so Fauxton, /_all_dbs
    and /_node/_local/_config — which rewrites the server config given
    admin credentials — 404 at the proxy while any number of per-vault
    databases pass. Verified against both sets of paths with caddy run
    against a stub backend;
  - the plugin's own E2EE carries the actual confidentiality: jupiter only
    ever stores ciphertext. Its passphrase is deliberately NOT in sops —
    it never leaves the clients, and pairing it with the server credential
    would defeat the point.

flush_interval -1 is required, not tuning: replication rides a continuous
_changes feed that caddy would otherwise buffer into a stall.

Storage sits on the array with RequiresMountsFor, since a CouchDB that
starts without /mnt/data would create an empty database on the eMMC and
LiveSync would replicate that emptiness back to every client. Logs go to
journald rather than the unrotated /var/log/couchdb.log, for the same
29G-eMMC reasons as the rest of jupiter.

The admin password reaches CouchDB as an [admins] ini fragment via
extraConfigFiles; services.couchdb.adminPass would have rendered it into
the world-readable store.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TLN5nkLBtCciD3ZnUwtw2b

hosts/neptun/configuration.nix

@@ -111,6 +111,62 @@
     reverse_proxy http://jupiter.orbit.sol:2283
   '';
 
+  # ---- Obsidian LiveSync (CouchDB on jupiter) ----
+  # Obsidian's mobile apps refuse cleartext HTTP and *.jupiter.sol cannot hold
+  # a publicly trusted cert, so the vault database is published here instead of
+  # staying on the LAN. That means a credentialed database on the open
+  # internet; two things keep it sane:
+  #
+  #  1. The plugin's end-to-end encryption, switched on BEFORE the first sync.
+  #     jupiter then stores only ciphertext, so a breach here is not a leak of
+  #     the notes themselves.
+  #  2. This allowlist. CouchDB serves far more than the replication API —
+  #     Fauxton (/_utils), /_all_dbs, and /_node/_local/_config, the last of
+  #     which REWRITES the server's config given admin credentials. Only the
+  #     paths the plugin actually speaks are proxied; everything else is
+  #     answered here and never reaches jupiter. Use the tailnet for the rest:
+  #     `curl http://jupiter.orbit.sol:5984/_utils/`.
+  #
+  # ONE DATABASE PER VAULT, and the matcher keys off CouchDB's own naming rule
+  # rather than listing them: every system endpoint begins with `_`, and a
+  # user-creatable database never can (CouchDB requires a lowercase letter
+  # first). So adding a vault needs no edit here. `_session` is the single
+  # underscore path let through, for cookie auth.
+  #
+  # The flip side of not listing them: a mistyped but otherwise LEGAL database
+  # name is proxied through and reaches CouchDB, which answers a real 404 the
+  # plugin can report. An ILLEGAL one — anything starting with a capital or an
+  # underscore — fails the matcher instead and gets caddy's 404, which carries
+  # no CORS headers and surfaces in Obsidian as a connection failure with no
+  # error message at all. If a new vault refuses to connect and the plugin
+  # says nothing, check the database name is lowercase first.
+  #
+  # Never point two vaults at one database: LiveSync merges them into a single
+  # file tree, which is not cleanly reversible.
+  #
+  # Known consequence: LiveSync's "Check database configuration" panel reads
+  # /_node/_local/_config and so reports the server as unconfigured from
+  # outside. Expected — that config is declarative in
+  # services/dev/obsidian-livesync.nix and is not the plugin's to patch.
+  #
+  # `flush_interval -1` is required, not tuning: replication rides a
+  # continuous _changes feed, which caddy would otherwise buffer — sync then
+  # stalls until the buffer fills (same reason vpn.mgaction.town sets it).
+  #
+  # No netcup edge-firewall change: this rides the 443 the other vhosts
+  # already use, unlike gitea's :2222.
+  services.caddy.virtualHosts."notes.mgaction.town".extraConfig = ''
+    @livesync path_regexp ^/(_session|[a-z][a-z0-9_$()+-]*)?(/.*)?$
+    handle @livesync {
+      reverse_proxy http://jupiter.orbit.sol:5984 {
+        flush_interval -1
+      }
+    }
+    handle {
+      respond 404
+    }
+  '';
+
   # ---- Hermes dashboard ----
   # Authentik-gated (hosts/mars/hermes-agent.nix has the OIDC config and the
   # "create the Authentik app" instructions — moved here from jupiter).