fix(live): window 'online' on server receive-time, not client clock

The player-count flapping was client clock skew: telemetry is stamped with the
game machine's DateTime.UtcNow (WebSocket.cs), and machines' clocks drift up to
~90s apart (proven: per-char offsets span -31s..+59s with steady 6s cadence; a
wrong server clock would shift all equally, so the SPREAD proves clients differ
from each other; a +59s future timestamp rules out lag). /live windowed on that
client timestamp, so characters whose clock sat near the 30s boundary blinked
in and out.

Fix: stamp each telemetry row with the server's receive-time (received_at) and
window the /live 'online' query on COALESCE(received_at, timestamp) instead of
the client timestamp. A coarse timestamp bound (10 min) is kept only for
TimescaleDB chunk pruning. Column added idempotently in init_db_async; COALESCE
falls back to the client timestamp for pre-migration rows. Verified on the live
DB: query valid, 8ms, equivalent pre-population. ~free CPU (one datetime.now()
per ~14 inserts/sec).

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

db_async.py

@@ -49,6 +49,11 @@ telemetry_events = Table(
     Column("cpu_pct", Float, nullable=True),
     Column("mem_handles", Integer, nullable=True),
     Column("latency_ms", Float, nullable=True),
+    # Server-side receive time. The `timestamp` column above is the CLIENT's
+    # self-reported wall clock and drifts up to ~90s across machines, so the
+    # "online" window must use this server-stamped value instead (see /live
+    # cache query). Nullable so pre-migration rows fall back to `timestamp`.
+    Column("received_at", DateTime(timezone=True), nullable=True),
 )
 # Composite index to accelerate Grafana queries filtering by character_name then ordering by timestamp
 Index(
@@ -256,6 +261,18 @@ async def init_db_async():
         print(
             f"Warning: failed to create composite index ix_telemetry_events_char_ts: {e}"
         )
+    # Add the server-receive-time column to existing deployments (idempotent).
+    # Used as the clock-skew-proof basis for the "online" window in /live.
+    try:
+        with engine.connect() as conn:
+            conn.execute(
+                text(
+                    "ALTER TABLE telemetry_events "
+                    "ADD COLUMN IF NOT EXISTS received_at TIMESTAMPTZ"
+                )
+            )
+    except Exception as e:
+        print(f"Warning: failed to add telemetry_events.received_at column: {e}")
     # Add retention and compression policies on the hypertable
     try:
         with engine.connect().execution_options(isolation_level="AUTOCOMMIT") as conn: