fix(discord-bridge): plafonul de cost e optional (COST_CAP_USD_DAY=off)

Costul raportat de CLI e pretul echivalent la API; pe abonament nu se
factureaza, deci plafonul zilnic oprea puntea fara motiv (5.71 / 5.00 USD).

- limits.parse_cap(): `off`/`none`/`nelimitat`/`0`/gol/gunoi => fara plafon
- stopped()/record_cost() nu mai opresc si nu mai alerteaza cand e dezactivat
- implicit devine `off`; /status arata „(fara plafon)"
- dashboard: /api/status si doctor nu mai crapa pe valoare ne-numerica
- README + ops/env.example explica de ce ramane off pe abonament

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01B29CApsP1JkSdjYaGaHpE7

proxmox/lxc171-claude-agent/discord-bridge/tests/test_limits.py

@@ -78,6 +78,27 @@ def test_plafon_de_cost_opreste_botul(store):
         lim.admit("u1")
 
 
+def test_plafonul_off_nu_opreste_nimic(store, state_dir):
+    import config
+    (state_dir / "env").write_text("COST_CAP_USD_DAY=off\n")
+    config.reload(state_dir)
+    alerte = []
+    lim = L.Limits(store, alerter=lambda *a, **k: alerte.append(a))
+    assert lim.cost_cap == 0.0 and not lim.cap_enabled
+    lim.record_cost(99.0, "1")
+    assert not lim.stopped() and lim.cost_remaining() == float("inf")
+    lim.admit("u1")            # nu ridica CostCapReached
+    assert alerte == []
+
+
+@pytest.mark.parametrize("raw, asteptat", [
+    ("off", 0.0), ("", 0.0), ("nelimitat", 0.0), ("0", 0.0), ("-3", 0.0),
+    ("5.00", 5.0), (" 2.5 ", 2.5), ("gunoi", 0.0), (None, 0.0),
+])
+def test_parse_cap(raw, asteptat):
+    assert L.parse_cap(raw, 0.0) == pytest.approx(asteptat)
+
+
 def test_plafon_alerteaza_o_singura_data(store):
     alerte = []
     lim = mkl(store, cost_cap=0.5, alerter=lambda *a, **k: alerte.append(a))