feat: scaffold Phase B + C (events, telemetry, plugins, Solid.js)

Phase B Step 6 — Solid.js incremental migration
  - vite-plugin-solid + solid-js + @solidjs/testing-library in package.json
  - vite.config.ts compiles src/components/solid/** as Solid TSX
  - tsconfig.json gains jsx: preserve / jsxImportSource: solid-js
  - lib/solidAdapter.ts wraps existing custom Stores as Solid signals
  - lib/solidMount.ts adapts Solid render to {mount,destroy} contract
  - components/solid/Badge.tsx (proof-of-concept leaf)
  - components/solid/ChannelListItem.tsx (store-subscribed leaf)
  - components/solid/Badge.test.tsx pipeline smoke test
  - components/solid/README.md documents the migration recipe

Phase B Step 7 — Event persistence layer
  - SQLite + Postgres migrations for the events table
  - sqlc query files for both engines
  - EventStore interface + SQLite raw-SQL impl + MemStore impl + pg stubs
  - ws.EventPersister: async batched writer (queue / flush / drain / drop)
  - ws.StartEventPruner: background retention pruner
  - hub persists every replay-buffer push and exposes reconnect-tier counters
  - serve.handleReconnect: tiered replay (buffer -> DB -> full re-sync)
  - EventPersistenceConfig + main.go wiring
  - event_persister_test.go covers batching / drops / drain

Phase B Step 8 — OpenTelemetry skeleton
  - Server/telemetry package with public Provider/Tracer/Meter/Counter API
  - telemetry_default.go (no-op build) + telemetry_otel.go (build tag otel)
  - telemetry/metrics.go declares the AppMetrics bundle
  - HTTPMiddleware mounted in Chi router (pass-through in default build)
  - PrometheusHandler optionally mounted at /metrics
  - Spans on MessageService.SendMessage, PermissionService.HasChannelPerm,
    ChannelService.ListVisibleChannels
  - Reconnect-tier counter wired into the global meter
  - TelemetryConfig defaults

Phase C Step 9 — Wazero plugin runtime skeleton
  - Server/plugin package: manifest parser, loader, registry, host APIs
    (commands, storage, events, http, ui), errors
  - sandbox_default.go (no-op) + sandbox_wazero.go (build tag wazero)
  - SQLite + Postgres migrations for plugins + plugin_kv tables
  - PluginStore interface + impls + pg stubs
  - plugin/examples/hello manifest + README
  - plugin_test.go covers manifest, loader, capability gating
  - api/plugins_handler.go admin REST surface, mounted under admin group
  - PluginsConfig + main.go wiring (disabled by default)
  - Client: lib/pluginBridge.ts iframe + postMessage host
  - Client: components/solid/PluginContainer.tsx Solid host component

Verification
  - Default build (no -tags) is intended to compile cleanly with no new
    third-party dependencies. The sandbox lacked Go 1.25.0 so go build
    could not run; PHASE_BC_LOCAL_TODO.md enumerates the local follow-up
    work (npm install, go mod tidy, sqlc-generate, real otel/wazero
    wiring, remaining service spans, full Solid migration).

Server/api/plugins_handler.go

@@ -0,0 +1,106 @@
+// Phase C Step 9 — Plugin admin REST surface.
+//
+// All endpoints are mounted under the existing AdminIPRestrict group so they
+// inherit the same network ACL as the rest of the admin panel. Authentication
+// is handled by the admin handler's middleware before this handler runs.
+package api
+
+import (
+	"net/http"
+	"strconv"
+
+	"github.com/go-chi/chi/v5"
+	"github.com/owncord/server/plugin"
+	"github.com/owncord/server/store"
+)
+
+// PluginAdminHandler exposes plugin lifecycle operations to the admin panel.
+type PluginAdminHandler struct {
+	registry *plugin.Registry
+	store    store.PluginStore
+}
+
+// NewPluginAdminHandler builds an http.Handler that the router can mount.
+// Pass a nil registry when plugin support is disabled — the handler then
+// reports an empty list and 503 on lifecycle calls.
+func NewPluginAdminHandler(registry *plugin.Registry, st store.PluginStore) http.Handler {
+	h := &PluginAdminHandler{registry: registry, store: st}
+	r := chi.NewRouter()
+	r.Get("/", h.list)
+	r.Post("/{id}/enable", h.enable)
+	r.Post("/{id}/disable", h.disable)
+	r.Delete("/{id}", h.uninstall)
+	return r
+}
+
+func (h *PluginAdminHandler) list(w http.ResponseWriter, r *http.Request) {
+	ctx := r.Context()
+	if h.store == nil {
+		writeJSON(w, http.StatusOK, []any{})
+		return
+	}
+	rows, err := h.store.ListPlugins(ctx)
+	if err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	writeJSON(w, http.StatusOK, rows)
+}
+
+func (h *PluginAdminHandler) enable(w http.ResponseWriter, r *http.Request) {
+	id, ok := parsePluginID(w, r)
+	if !ok {
+		return
+	}
+	if h.registry == nil {
+		http.Error(w, "plugin runtime disabled", http.StatusServiceUnavailable)
+		return
+	}
+	if err := h.registry.EnablePlugin(r.Context(), id); err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (h *PluginAdminHandler) disable(w http.ResponseWriter, r *http.Request) {
+	id, ok := parsePluginID(w, r)
+	if !ok {
+		return
+	}
+	if h.registry == nil {
+		http.Error(w, "plugin runtime disabled", http.StatusServiceUnavailable)
+		return
+	}
+	if err := h.registry.DisablePlugin(r.Context(), id); err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func (h *PluginAdminHandler) uninstall(w http.ResponseWriter, r *http.Request) {
+	id, ok := parsePluginID(w, r)
+	if !ok {
+		return
+	}
+	if h.registry == nil {
+		http.Error(w, "plugin runtime disabled", http.StatusServiceUnavailable)
+		return
+	}
+	if err := h.registry.UninstallPlugin(r.Context(), id); err != nil {
+		http.Error(w, err.Error(), http.StatusInternalServerError)
+		return
+	}
+	w.WriteHeader(http.StatusNoContent)
+}
+
+func parsePluginID(w http.ResponseWriter, r *http.Request) (int64, bool) {
+	idStr := chi.URLParam(r, "id")
+	id, err := strconv.ParseInt(idStr, 10, 64)
+	if err != nil || id <= 0 {
+		http.Error(w, "invalid plugin id", http.StatusBadRequest)
+		return 0, false
+	}
+	return id, true
+}