Refactor dev services to support homeserver choice (Continuwuity or Synapse)

The dev environment previously hardcoded Synapse (bundled with Postgres
and Element Web) in a monolithic etc/services/core/ directory.

With Continuwuity now available as a lighter alternative (no external DB),
this refactors the service layout so developers choose their homeserver
once and everything derives from that choice. Continuwuity is the new
default for its smaller footprint.

Key changes:
- Break etc/services/core/ into etc/services/synapse/ and
  etc/services/element-web/, each with their own compose.yml
- Add `homeserver` variable in justfile (reads var/homeserver,
  defaults to continuwuity)
- Add `homeserver-init` recipe to persist the choice
- Use placeholders (__HOMESERVER_SERVER_NAME__, __HOMESERVER_URL__,
  __HOMESERVER_CLIENT_URL__) in config templates, resolved at
  prepare time based on the chosen homeserver
- Make services-start/stop/prepare/tail-logs delegate to the chosen
  homeserver's recipes + element-web
- Make users-prepare delegate to {homeserver}-users-prepare
- Update docs/development.md for the new homeserver choice flow

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

etc/app/config.yml.dist

@@ -1,7 +1,7 @@
 homeserver:
   # The canonical homeserver domain name
-  server_name: synapse.127.0.0.1.nip.io
-  url: http://synapse.127.0.0.1.nip.io:42020
+  server_name: __HOMESERVER_SERVER_NAME__
+  url: __HOMESERVER_URL__
 
 user:
   mxid_localpart: baibot
@@ -45,7 +45,7 @@ room:
 access:
   # Space-separated list of MXID patterns which specify who is an admin.
   admin_patterns:
-    - "@admin:synapse.127.0.0.1.nip.io"
+    - "@admin:__HOMESERVER_SERVER_NAME__"
 
 persistence:
   # This is unset here, because we expect the configuration to come from an environment variable (BAIBOT_PERSISTENCE_DATA_DIR_PATH).
@@ -157,7 +157,7 @@ initial_global_config:
   # Space-separated list of MXID patterns which specify who can use the bot.
   # By default, we let anyone on the homeserver use the bot.
   user_patterns:
-    - "@*:synapse.127.0.0.1.nip.io"
+    - "@*:__HOMESERVER_SERVER_NAME__"
 
 # Controls logging.
 #