feat: add Docker support with self-hosted and hosted presets

Multi-stage Dockerfile (Node 22 Alpine), docker-compose for self-hosted
and hosted deployments, CI workflow for GHCR publishing, runtime env var
substitution, and cron sidecar with supercronic.

Aligns hosted extension preset with dev config (enable-banking,
ai-categorization, ai-chat, email).

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

docker-compose.build.yml

@@ -0,0 +1,9 @@
+# Override for building from source instead of pulling from GHCR.
+# Usage: docker compose -f docker-compose.yml -f docker-compose.build.yml up --build
+services:
+  app:
+    image: !reset null
+    build:
+      context: .
+      args:
+        EXTENSIONS_PRESET: self-hosted