CB-599: surface a capacity refusal's reason instead of a bare 500

PlacementException extends IllegalStateException, which neither BridgedApp
nor BridgeMcp's spawn catch blocks handled, so a maxLoad/quarantine/
all-exhausted refusal fell through to a blank 500 on REST and lost its
message on MCP. Catch it on both surfaces, ahead of the unrelated
IllegalArgumentException(unknown_profile) mapping, and return its message
structured: REST as {"error":"no_capacity","detail":...} with status 503,
MCP as an isError result prefixed "no capacity: ...".

bridged/src/main/java/dev/ltms/bridged/mcp/BridgeMcp.java

@@ -15,6 +15,7 @@ import dev.ltms.bridged.msg.MessageService;
 import dev.ltms.bridged.msg.Rendezvous;
 import dev.ltms.bridged.peer.PeerUnreachableException;
 import dev.ltms.bridged.placement.BackendQuarantine;
+import dev.ltms.bridged.placement.PlacementException;
 import dev.ltms.bridged.session.SessionManager;
 import dev.ltms.bridged.session.MemberSession;
 import dev.ltms.bridged.session.WorktreeRequest;
@@ -692,6 +693,10 @@ public final class BridgeMcp {
             return text(json(memberView(member)));
         } catch (GuardException e) {
             return error("subscription boundary: " + e.getMessage());
+        } catch (PlacementException e) {
+            // CB-599: no candidate had capacity (maxLoad, quarantine, or all-exhausted) — distinct
+            // from "profile does not exist" below.
+            return error("no capacity: " + e.getMessage());
         } catch (IllegalArgumentException e) {
             return error(e.getMessage()); // unknown / no-default profile, or a refused resumeSessionId
         } catch (PeerUnreachableException e) {

bridged/src/main/java/dev/ltms/bridged/rest/BridgedApp.java

@@ -13,6 +13,7 @@ import dev.ltms.bridged.herdr.HerdrClient;
 import dev.ltms.bridged.herdr.HerdrException;
 import dev.ltms.bridged.inject.MemberPresence;
 import dev.ltms.bridged.peer.PeerUnreachableException;
+import dev.ltms.bridged.placement.PlacementException;
 import dev.ltms.bridged.msg.MessageService;
 import dev.ltms.bridged.session.SessionManager;
 import dev.ltms.bridged.peer.MemberRole;
@@ -292,6 +293,11 @@ public final class BridgedApp {
             ctx.status(201).json(view(member));
         } catch (GuardException e) {
             ctx.status(403).json(Map.of("error", "subscription_boundary", "detail", e.getMessage()));
+        } catch (PlacementException e) {
+            // CB-599: no candidate had capacity (maxLoad, quarantine, or all-exhausted) — a benign,
+            // likely-transient refusal, distinct from "profile does not exist" below. 503: the
+            // request was valid and will likely succeed later.
+            ctx.status(503).json(Map.of("error", "no_capacity", "detail", e.getMessage()));
         } catch (IllegalArgumentException e) {
             ctx.status(400).json(Map.of("error", "unknown_profile", "detail", e.getMessage()));
         } catch (PeerUnreachableException e) {