Add the schulcloud CLI, and document the split

The CLI talks only to the Pi's /api surface and holds no Schulcloud
credential — only the same bearer token the Claude connector uses. That
is not layering for its own sake: a Schulcloud session dies after two
hours idle and a CLI process lives for seconds, so a CLI with its own
token would be dead most times you reached for it. Routing through the
Pi means one session, one keepalive, one monthly cookie paste.

sync is a one-way mirror, which follows from the data rather than from
scope-cutting: file records are immutable upstream, so there is no
versioning, no conflict resolution and no merge. State is keyed by file
record id with the path as derived output, so an upstream rename moves
the local file instead of duplicating it — verified against the live
server. Verification is size-only because the download endpoint exposes
no ETag and Schulcloud publishes no hash; size still catches the failure
that happens, a truncated download. Downloads land on a .part neighbour
and are renamed, so an interrupted run leaves no half-file that a later
run mistakes for complete. Deletions are reported but not propagated —
a teacher removing a worksheet is no reason to destroy the student's
copy — with --prune to opt in.

what_changed now clamps to the oldest stored generation instead of
refusing, and says it did: "what's new this week" is a reasonable
question to ask a two-day-old index.

Two build bugs caught by the checks rather than by luck: the smoke
harness constructed the app without services, so the index-backed tools
were never exercised; and the Docker build could not see
scripts/copy-assets.mjs, so the image would have shipped without
migrations and silently degraded to live-only.

67 unit tests (9 needing Postgres), smoke green both ways — 34 checks
with an index, 32 without, because graceful degradation is a supported
mode and not a fallback nobody runs.

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

.env.example

@@ -26,6 +26,27 @@ MCP_AUTH_TOKEN=
 PORT=8080
 BIND_HOST=0.0.0.0
 
+# ---------------------------------------------------------------------------
+# Index and file mirror (optional — without these the server runs live-only:
+# search crawls on every call, and the CLI's /api surface is unavailable)
+# ---------------------------------------------------------------------------
+
+# Postgres for crawl generations, full-text search and the file mirror index.
+# On the Pi, point this at the existing instance with its own database and user.
+DATABASE_URL=postgresql://schulcloud:schulcloud@postgres:5432/schulcloud
+
+# Where mirrored file bytes are stored. Needs to be writable by the container.
+# MIRROR_DIR=/data/mirror
+
+# Files larger than this are indexed as metadata but not mirrored; they are
+# still downloadable, proxied live. Default 64 MiB.
+# MIRROR_MAX_BYTES=67108864
+
+# How often to re-crawl on a timer, in ms. Default 21600000 (6h). 0 = on demand
+# only. A re-crawl of unchanged content downloads nothing, because Schulcloud
+# file records are immutable.
+# CRAWL_INTERVAL_MS=21600000
+
 # ---------------------------------------------------------------------------
 # Limits (optional — sensible defaults are built in)
 # ---------------------------------------------------------------------------