feat: persistent per-application config with GET/PUT endpoints

Add application_config table (V4 migration), repository, and REST
controller. GET /api/v1/config/{app} returns config, PUT saves and
pushes CONFIG_UPDATE to all LIVE agents via SSE. UI tracing toggle
now uses config API instead of direct SET_TRACED_PROCESSORS command.
Tracing store syncs with server config on load.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cameleer3-server-app/src/main/java/com/cameleer3/server/app/controller/ApplicationConfigController.java

@@ -0,0 +1,104 @@
+package com.cameleer3.server.app.controller;
+
+import com.cameleer3.common.model.ApplicationConfig;
+import com.cameleer3.server.app.storage.PostgresApplicationConfigRepository;
+import com.cameleer3.server.core.agent.AgentCommand;
+import com.cameleer3.server.core.agent.AgentInfo;
+import com.cameleer3.server.core.agent.AgentRegistryService;
+import com.cameleer3.server.core.agent.AgentState;
+import com.cameleer3.server.core.agent.CommandType;
+import com.fasterxml.jackson.core.JsonProcessingException;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import io.swagger.v3.oas.annotations.Operation;
+import io.swagger.v3.oas.annotations.responses.ApiResponse;
+import io.swagger.v3.oas.annotations.tags.Tag;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.http.ResponseEntity;
+import org.springframework.security.core.Authentication;
+import org.springframework.web.bind.annotation.*;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Per-application configuration management.
+ * Agents fetch config at startup; the UI modifies config which is persisted and pushed to agents via SSE.
+ */
+@RestController
+@RequestMapping("/api/v1/config")
+@Tag(name = "Application Config", description = "Per-application observability configuration")
+public class ApplicationConfigController {
+
+    private static final Logger log = LoggerFactory.getLogger(ApplicationConfigController.class);
+
+    private final PostgresApplicationConfigRepository configRepository;
+    private final AgentRegistryService registryService;
+    private final ObjectMapper objectMapper;
+
+    public ApplicationConfigController(PostgresApplicationConfigRepository configRepository,
+                                       AgentRegistryService registryService,
+                                       ObjectMapper objectMapper) {
+        this.configRepository = configRepository;
+        this.registryService = registryService;
+        this.objectMapper = objectMapper;
+    }
+
+    @GetMapping("/{application}")
+    @Operation(summary = "Get application config",
+            description = "Returns the current configuration for an application. Returns defaults if none stored.")
+    @ApiResponse(responseCode = "200", description = "Config returned")
+    public ResponseEntity<ApplicationConfig> getConfig(@PathVariable String application) {
+        return ResponseEntity.ok(
+                configRepository.findByApplication(application)
+                        .orElse(defaultConfig(application)));
+    }
+
+    @PutMapping("/{application}")
+    @Operation(summary = "Update application config",
+            description = "Saves config and pushes CONFIG_UPDATE to all LIVE agents of this application")
+    @ApiResponse(responseCode = "200", description = "Config saved and pushed")
+    public ResponseEntity<ApplicationConfig> updateConfig(@PathVariable String application,
+                                                           @RequestBody ApplicationConfig config,
+                                                           Authentication auth) {
+        String updatedBy = auth != null ? auth.getName() : "system";
+
+        config.setApplication(application);
+        ApplicationConfig saved = configRepository.save(application, config, updatedBy);
+
+        int pushed = pushConfigToAgents(application, saved);
+        log.info("Config v{} saved for '{}', pushed to {} agent(s)", saved.getVersion(), application, pushed);
+
+        return ResponseEntity.ok(saved);
+    }
+
+    private int pushConfigToAgents(String application, ApplicationConfig config) {
+        String payloadJson;
+        try {
+            payloadJson = objectMapper.writeValueAsString(config);
+        } catch (JsonProcessingException e) {
+            log.error("Failed to serialize config for push", e);
+            return 0;
+        }
+
+        List<AgentInfo> agents = registryService.findAll().stream()
+                .filter(a -> a.state() == AgentState.LIVE)
+                .filter(a -> application.equals(a.application()))
+                .toList();
+
+        for (AgentInfo agent : agents) {
+            registryService.addCommand(agent.id(), CommandType.CONFIG_UPDATE, payloadJson);
+        }
+        return agents.size();
+    }
+
+    private static ApplicationConfig defaultConfig(String application) {
+        ApplicationConfig config = new ApplicationConfig();
+        config.setApplication(application);
+        config.setVersion(0);
+        config.setMetricsEnabled(true);
+        config.setSamplingRate(1.0);
+        config.setTracedProcessors(Map.of());
+        return config;
+    }
+}