feat(telegram): bot support relay — per-user forum topics

Users who DM the bot anything but /start are relayed into a private
forum supergroup, one topic per user. Operators (the chat's admins)
reply in the topic and the bot copies it back to the user; an info card
opening each topic carries a Block/Unblock toggle and a Clear button.
State is a small JSON file on a new /data volume — the bot host has no
database. Off by default (TELEGRAM_SUPPORT_CHAT_ID=0): the prod bot is
unchanged until the operator sets the chat id and adds the bot as a
forum admin.

- internal/support: concurrency-safe JSON store (topic map, block list,
  relayed message ids) with field-targeted mutators and atomic save
- bot/support.go: relay both ways via copyMessage, short-TTL admin
  cache, callback buttons, per-user topic-create lock, loop guard
  (skip the bot's own posts), reopen a deleted topic on the next message
- config + compose + CI/prod-deploy: TELEGRAM_SUPPORT_CHAT_ID per
  contour + TELEGRAM_SUPPORT_STATE_DIR; bot-state named volume; /data
  pre-owned by UID 65532 so a fresh volume is writable under distroless
- docs: ARCHITECTURE §15 + decision record, FUNCTIONAL (+ru), README

platform/telegram/internal/config/config_test.go

@@ -151,6 +151,48 @@ func TestLoadBotChatAndPromo(t *testing.T) {
 	})
 }
 
+// TestLoadBotSupportRelay verifies the support-relay chat id parses, the state
+// directory defaults to /data, and the relay is disabled (chat id 0) by default.
+func TestLoadBotSupportRelay(t *testing.T) {
+	t.Run("parsed", func(t *testing.T) {
+		setBotRequired(t)
+		t.Setenv("TELEGRAM_SUPPORT_CHAT_ID", "-1001234567890")
+		t.Setenv("TELEGRAM_SUPPORT_STATE_DIR", "/srv/state")
+		c, err := LoadBot()
+		if err != nil {
+			t.Fatalf("LoadBot: %v", err)
+		}
+		if c.SupportChatID != -1001234567890 {
+			t.Errorf("SupportChatID = %d, want -1001234567890", c.SupportChatID)
+		}
+		if c.SupportStateDir != "/srv/state" {
+			t.Errorf("SupportStateDir = %q, want /srv/state", c.SupportStateDir)
+		}
+	})
+
+	t.Run("defaults", func(t *testing.T) {
+		setBotRequired(t)
+		c, err := LoadBot()
+		if err != nil {
+			t.Fatalf("LoadBot: %v", err)
+		}
+		if c.SupportChatID != 0 {
+			t.Errorf("SupportChatID = %d, want 0 (relay disabled)", c.SupportChatID)
+		}
+		if c.SupportStateDir != "/data" {
+			t.Errorf("SupportStateDir = %q, want /data", c.SupportStateDir)
+		}
+	})
+
+	t.Run("malformed chat id rejected", func(t *testing.T) {
+		setBotRequired(t)
+		t.Setenv("TELEGRAM_SUPPORT_CHAT_ID", "not-a-number")
+		if _, err := LoadBot(); err == nil {
+			t.Fatal("LoadBot: expected an error for a malformed support chat id, got nil")
+		}
+	})
+}
+
 // TestLoadRejectsUnsupportedExporter verifies an exporter outside the supported set
 // fails validation (the validator path).
 func TestLoadRejectsUnsupportedExporter(t *testing.T) {