T11.4: usage export endpoint (raw + sanitized CSV)

CR-019 / wave 11. Adds GET /api/usage/export, reusing _usage_query()'s
scoped filters (from/to/project_id/username/tool) and the same
require_user_manager gate as /api/usage/summary. Two modes:

- raw (default): real usernames, for internal admin use.
- sanitize=true: usernames replaced with an HMAC-SHA256 pseudonym
  (keyed with auth.SECRET_KEY, 16 hex chars, 'u_' prefix) so the file
  can be fed into PowerBI or another external reporting tool without
  carrying real identities. HMAC chosen over a plain hash since the
  username space is small enough to brute-force a bare digest.

Both modes emit at, username, project_id, tool, event as columns and
deliberately omit the detail JSON column in both modes to avoid an
identity leak riding along inside free-form detail data. Response is
returned with a Content-Disposition: attachment header and a filename
that encodes mode + date.

Verified locally against a throwaway SQLite DB with two seeded users
and four seeded UsageEvent rows:
  - raw export contains the real usernames and matches the summary
    endpoint's event_count for the same session state
  - sanitized export contains no real username or email anywhere in
    the file body, across two independently-issued export calls
  - the same real user maps to the same pseudonym both within one
    export and across the two separate export calls
  - raw and sanitized rows line up 1:1 on at/tool/event for the same
    filter set
  - the tool= filter narrows the export the same way it narrows the
    summary
  - a plain project_user is refused with 403; an unauthenticated
    request is refused with 401
  - full smoke test (27/27) and seed_demo.py both still pass

server/app.py

@@ -9,6 +9,10 @@ Run (prod):    gunicorn -k uvicorn.workers.UvicornWorker -b 127.0.0.1:8000 serve
 Interactive docs: http://<host>/api/docs
 """
 import base64
+import csv
+import hashlib
+import hmac
+import io
 import logging
 import os
 import re
@@ -3509,6 +3513,59 @@ def usage_summary(
     }
 
 
+def _pseudonym(username: str) -> str:
+    """A stable per-user id for the sanitized export — the SAME input always
+    produces the SAME output, within one export and across separate export
+    runs, so an external system (Power BI or similar) can still group and
+    trend "by user" without ever receiving a real name. HMAC rather than a
+    plain hash: a plain sha256(username) is trivially reversed against a
+    wordlist of the handful of usernames this app actually has; keying it
+    with AUTH_SECRET_KEY (already a real secret, already required in
+    production — see auth.py) means recovering a username from its
+    pseudonym requires the signing key, not just guessing."""
+    digest = hmac.new(auth.SECRET_KEY.encode(), username.encode(), hashlib.sha256).hexdigest()
+    return "u_" + digest[:16]
+
+
+@app.get("/api/usage/export")
+def usage_export(
+    date_from: Optional[str] = Query(None, alias="from"),
+    date_to: Optional[str] = Query(None, alias="to"),
+    project_id: Optional[str] = Query(None),
+    username: Optional[str] = Query(None),
+    tool: Optional[str] = Query(None),
+    sanitize: bool = Query(False),
+    caller: models.User = Depends(require_user_manager),
+    db: Session = Depends(get_db),
+):
+    """CR-019. Same gate, same filters, same underlying row set as
+    usage_summary() (_usage_query) — the export can never show a different
+    slice of data than what the console counted for the same filters.
+
+    sanitize=true replaces `username` with a stable pseudonym (_pseudonym)
+    and — deliberately — the `detail` column is not exported in EITHER mode.
+    Every event this app writes today (login, page_open) leaves `detail`
+    empty, so this costs nothing now, but it also means a future event type
+    that DOES populate `detail` can't accidentally leak a real name into a
+    sanitized file through a column nobody thought to scrub. If `detail`
+    is ever needed in the export, it has to be sanitized explicitly, not
+    assumed safe because the rest of the row was."""
+    rows = db.scalars(_usage_query(db, caller, date_from, date_to, project_id, username, tool)).all()
+    buf = io.StringIO()
+    w = csv.writer(buf)
+    w.writerow(["at", "username", "project_id", "tool", "event"])
+    for e in rows:
+        who = _pseudonym(e.username) if sanitize else e.username
+        w.writerow([models._iso(e.at), who, e.project_id or "", e.tool, e.event])
+    filename = "usage_export_%s_%s.csv" % (
+        "sanitized" if sanitize else "raw", datetime.now(timezone.utc).strftime("%Y%m%d"),
+    )
+    return Response(
+        content=buf.getvalue(), media_type="text/csv",
+        headers={"Content-Disposition": f'attachment; filename="{filename}"'},
+    )
+
+
 @app.get("/api/comments")
 def list_comments(
     source: Optional[str] = Query(None),