add Dockerfile + deployment docs; make config dbPath optional

node:24-bookworm-slim image with git for the read-only vault endpoints,
non-root user with system-wide git safe.directory (mounted vaults are
usually owned by a host uid), /config + /data + /vaults mount
conventions baked in via BRAINDUMP_CONFIG/BRAINDUMP_DB, and an
/api/health healthcheck. README gains a Deployment section (mounts,
claude-sdk skill/auth requirements in-container, compose example,
reverse-proxy SSE buffering notes).

dbPath in the config file now defaults to ./braindump.sqlite instead of
being required — it was validated before the BRAINDUMP_DB env override
was applied, so the image's baked-in db path couldn't rescue a config
that omitted it.

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

.dockerignore

@@ -0,0 +1,8 @@
+node_modules
+dist
+.git
+*.sqlite
+*.sqlite-*
+config.jsonc
+Dockerfile
+.dockerignore