build: docker image and compose

Dockerfile on oven/bun:1.3 with ffmpeg, git, unzip and python3-pymupdf, a
production install, non-root user, /data volume and a health check;
docker-compose.yml (project stc) with an optional postgres:17 profile and
read-only course and import mounts; docs/operations.md with configuration,
mounts, security headers and the measured idle memory (101.4 MiB for app plus
Postgres after 5 minutes idle).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,13 @@
+.git
+.worktrees
+.superpowers
+node_modules
+**/node_modules
+data
+reports
+research_notes
+docs
+**/test
+fixtures
+*.log
+.env