fix: stored logs keep bad rows, erased accounts and unconsented queries out

Deep-review round 1 of E22.

- A NUL character (`GET /assets/%00` logs the decoded path) made Postgres
  refuse the row and with it the sink's whole batch, every user's rows
  included. `to_row` replaces NUL in every text column and in attrs.
- An erased account's lines kept arriving after purge_user's DELETE: the
  erasing request's own access line, a queued row, a background task.
  `logs.forget_user` runs first; later rows are stored without the account,
  and the check and the insert share a lock with the erasure.
- An admin setting the whole process to TRACE logged every user's query
  string. The query is now logged only under the user's own Teljes consent,
  and decoded before redaction, so `%40` no longer hides an address.
- The browser log ingest has an hourly storage budget per account (5 MiB,
  one notice when spent) and reads only a JSON Content-Type, so a
  same-site text/plain form post cannot write into a user's log.
  RateLimiter takes a per-attempt cost for the byte budget.
- The root level is applied under the elevation lock: two concurrent
  changes could leave it at the base while an elevation was active.
- Tests for the running sink (level gate, drain on stop) and for the
  admin log level applied at runtime and at boot.

backend/app/routes/diagnostics.py

@@ -7,8 +7,9 @@ the body), and the request id each event carries, which is what joins a browser
 line of the same request.
 
 Nothing from the body is trusted as-is:
-  * the whole body is capped before it is parsed, and so are the number of events and each event's
-    attributes;
+  * only a JSON Content-Type is read, and the whole body is capped before it is parsed, and so are
+    the number of events and each event's attributes;
+  * the pace is capped per account, and so is what an account may store per hour;
   * everything is redacted AGAIN with the server's own rules — the client redacts at capture, but a
     modified client would not;
   * an event below the consented level is dropped (a Részletes consent does not store what only
@@ -40,6 +41,13 @@ ATTRS_MAX_BYTES = 16 * 1024
 # The client flushes every few seconds and when its buffer fills; this is room for a burst, not a
 # pace anything legitimate keeps up for a minute.
 _batches = RateLimiter(max_events=30, window_seconds=60)
+# What one account may STORE per hour. The pace cap alone still lets a self-consented account write
+# ~15 MiB a minute into the one shared database, kept for the whole retention window; a real session
+# at Teljes stores a few hundred kB an hour. Past the budget a batch is accepted and not stored (a
+# refusal would only make the client resend it), and one notice per hour says so.
+STORED_BYTES_PER_HOUR = 5 * 1024 * 1024
+_stored_bytes = RateLimiter(max_events=STORED_BYTES_PER_HOUR, window_seconds=3600)
+_budget_notices = RateLimiter(max_events=1, window_seconds=3600)
 
 ClientLevel = Literal["TRACE", "DEBUG", "INFO", "WARNING", "ERROR", "CRITICAL"]
 
@@ -73,7 +81,14 @@ async def _capped_body(request: Request) -> bytes:
 
 async def _parsed_events(request: Request) -> ClientEventsIn:
     """The body as the model — parsed only after the size cap, which is why this is a dependency and
-    not a plain body parameter (FastAPI reads a declared body in full before anything else runs)."""
+    not a plain body parameter (FastAPI reads a declared body in full before anything else runs).
+
+    JSON only, by its Content-Type, as a declared body would be. Parsing any body as JSON would let a
+    CORS-simple `text/plain` form post from a same-site page write events into a signed-in user's
+    log; the app's own client always sends `application/json`, the unload beacon included."""
+    media_type = request.headers.get("content-type", "").split(";")[0].strip().lower()
+    if media_type != "application/json" and not media_type.endswith("+json"):
+        raise HTTPException(status_code=415, detail={"code": "json_required"})
     raw = await _capped_body(request)
     try:
         return ClientEventsIn.model_validate_json(raw)
@@ -129,7 +144,33 @@ def ingest_events(
         for item in body.events
         if logs.SEVERITY[item.level] >= floor
     ]
-    if rows:
-        db.execute(insert(LogEvent), rows)
+    if not rows:
+        return {"ok": True, "stored": 0}
+    size = sum(len(json.dumps(row, ensure_ascii=False, default=str).encode()) for row in rows)
+    stored = len(rows)
+    if not _stored_bytes.allow(str(user.id), size):
+        stored = 0
+        if not _budget_notices.allow(str(user.id)):
+            return {"ok": True, "stored": 0}
+        rows = [_budget_notice(user, now)]
+    # Under the erasure guard until the commit: an account erased meanwhile gets no rows back.
+    with logs.erasure_guard():
+        db.execute(insert(LogEvent), logsink.without_erased(rows))
         db.commit()
-    return {"ok": True, "stored": len(rows)}
+    return {"ok": True, "stored": stored}
+
+
+def _budget_notice(user: User, now: datetime) -> dict[str, Any]:
+    return logsink.to_row(
+        {
+            "ts": now,
+            "level": "WARNING",
+            "sev": logs.SEVERITY["WARNING"],
+            "source": "web",
+            "logger": "web",
+            "event": "diagnostics.budget_spent",
+            "msg": "The browser log passed its hourly storage budget; the rest of the hour is not kept",
+            "attrs": {"budget_bytes": STORED_BYTES_PER_HOUR},
+            "user_id": user.id,
+        }
+    )