feat(obsidian): add CouchDB stack for Obsidian LiveSync sync

Self-hosted sync backend for the Obsidian LiveSync plugin, following
https://blog.admin-intelligence.de/selbsthosting-von-obsidian-mit-couchdb-kostenlose-synchronisation-einrichten/.
local.ini.example carries the required chttpd/cors config; local.ini itself
is gitignored since CouchDB rewrites it in place (hashed admin password,
node uuid) on first start, so a tracked copy would never stay clean. The
config also has to be mounted read-write, not :ro: CouchDB's entrypoint
chowns everything under /opt/couchdb and aborts under set -e if that fails.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

obsidian/local.ini.example

@@ -0,0 +1,35 @@
+# CouchDB settings required by the Obsidian LiveSync plugin.
+# Mounted into /opt/couchdb/etc/local.d/ (files here override the image's
+# default.ini and are picked up on container start, no admin-UI setup needed).
+#
+# Copy this to local.ini before first start:
+#   cp local.ini.example local.ini
+# local.ini is gitignored: CouchDB rewrites it at runtime (hashed admin
+# password, generated node uuid), so it can't be a tracked static file.
+#
+# Mounted read-write on purpose: the entrypoint chowns everything under
+# /opt/couchdb to the couchdb user on start and aborts if that fails, so a
+# :ro bind mount here breaks the container. It'll take ownership of this
+# file on the host after first start; that's expected.
+#
+# Reference: https://blog.admin-intelligence.de/selbsthosting-von-obsidian-mit-couchdb-kostenlose-synchronisation-einrichten/
+
+[couchdb]
+single_node = true
+max_document_size = 50000000
+
+[chttpd]
+require_valid_user = true
+enable_cors = true
+max_http_request_size = 4294967296
+
+[chttpd_auth]
+require_valid_user = true
+
+[httpd]
+WWW-Authenticate = Basic realm="couchdb"
+enable_cors = true
+
+[cors]
+credentials = true
+origins = app://obsidian.md,capacitor://localhost,http://localhost