feat(admin): background markdown editor with character cap

Phase 8c Task 5. Server Action caps at 40000 chars on write, owner-
only via requireOwner(). Client component: one textarea, live
counter, explicit Save button (no auto-save), repo URL rendered as
a 'see also' link rather than a tool call.

src/app/actions/project-background.ts

@@ -0,0 +1,44 @@
+"use server";
+
+import { revalidatePath } from "next/cache";
+
+import { getAdminPb } from "@/lib/pb/admin";
+import { requireSession } from "@/lib/auth/session";
+import type { ProjectRecord } from "@/lib/pb/types";
+
+const FIELD_CAP = 40_000;
+
+/** Owner-only entry point. Same pattern as log-work.ts. */
+async function requireOwner() {
+  const session = await requireSession();
+  if (session.role !== "owner") throw new Error("Not found");
+  return session;
+}
+
+/**
+ * Save the project's background markdown.
+ *
+ * The 40000 field cap is the trip-wire: at that point the doc is too big
+ * for whole-doc prompting and we should revisit section-aware retrieval.
+ * Until then, the cap is the only validation we do here — anything the
+ * developer writes is the developer's responsibility.
+ */
+export async function setProjectBackgroundMdAction(input: {
+  projectId: string;
+  backgroundMd: string;
+}): Promise<void> {
+  await requireOwner();
+
+  const text = (input.backgroundMd ?? "").slice(0, FIELD_CAP);
+
+  const pb = await getAdminPb();
+  const project = await pb
+    .collection("projects")
+    .getOne<ProjectRecord>(input.projectId);
+
+  await pb.collection("projects").update(project.id, {
+    background_md: text,
+  });
+
+  revalidatePath(`/admin/projects/${input.projectId}`);
+}

src/components/admin/background-md-editor.tsx

@@ -0,0 +1,128 @@
+"use client";
+
+import { useState, useTransition } from "react";
+import { Save } from "lucide-react";
+
+import { setProjectBackgroundMdAction } from "@/app/actions/project-background";
+
+const FIELD_CAP = 40_000;
+
+/**
+ * The per-project background markdown editor.
+ *
+ * One textarea, live character counter, no preview pane. A preview pane
+ * invites half-maintained doc; the editor is the source of truth and the
+ * wizard is the consumer. Save is its own button — auto-save on every
+ * keystroke would hammer the admin pb with no upside.
+ *
+ * The helper text mentions the GitHub link as a "see also" — the URL is
+ * not turned into a tool call; the developer is expected to embed file
+ * paths in the markdown itself.
+ */
+export function BackgroundMdEditor({
+  projectId,
+  initial,
+  repoUrl,
+}: {
+  projectId: string;
+  initial: string;
+  repoUrl: string;
+}) {
+  const [value, setValue] = useState(initial);
+  const [pending, startTransition] = useTransition();
+  const [saved, setSaved] = useState<boolean>(initial.length > 0);
+  const [error, setError] = useState<string | null>(null);
+
+  const dirty = value !== initial;
+  const over = value.length > FIELD_CAP;
+
+  function save() {
+    setError(null);
+    startTransition(async () => {
+      try {
+        await setProjectBackgroundMdAction({
+          projectId,
+          backgroundMd: value,
+        });
+        setSaved(true);
+      } catch (e) {
+        setError(e instanceof Error ? e.message : "Could not save");
+      }
+    });
+  }
+
+  const field =
+    "w-full rounded-lg border border-border-subtle bg-surface px-3 py-2 font-mono text-xs text-text-secondary outline-none focus:border-brand";
+
+  return (
+    <section className="space-y-3 rounded-[var(--radius-tile)] border border-border-subtle p-4">
+      <div>
+        <h2 className="text-sm font-semibold text-text-primary">
+          Background notes for the assistant
+        </h2>
+        <p className="mt-0.5 text-xs text-text-muted">
+          Plain markdown describing the app: what each section does today,
+          where it is allowed to go, what you would push back on. Sent into
+          every wizard prompt so the assistant answers clients as if it
+          knew the codebase.
+        </p>
+      </div>
+
+      <textarea
+        rows={12}
+        value={value}
+        onChange={(event) => {
+          setValue(event.target.value);
+          setSaved(false);
+        }}
+        placeholder={"## Quotes\nPriced in src/services/quote/create.ts.\nDiscount cap 15% without owner approval.\n"}
+        className={field}
+      />
+
+      <div className="flex items-center gap-3 text-xs text-text-muted">
+        <span className={over ? "text-accent" : ""}>
+          <span className="tabular">{value.length}</span> /{" "}
+          <span className="tabular">{FIELD_CAP}</span> characters
+        </span>
+        {over && (
+          <span>
+            Trim this — anything past the cap is not sent into prompts.
+          </span>
+        )}
+        {repoUrl && (
+          <span className="ml-auto">
+            See also:{" "}
+            <a
+              href={repoUrl}
+              target="_blank"
+              rel="noreferrer"
+              className="text-text-secondary underline-offset-2 hover:underline"
+            >
+              {repoUrl.replace(/^https?:\/\//, "")}
+            </a>
+          </span>
+        )}
+      </div>
+
+      <div className="flex items-center gap-3 border-t border-border-subtle pt-3">
+        <button
+          type="button"
+          disabled={pending || !dirty || over}
+          onClick={save}
+          className="inline-flex items-center gap-1.5 rounded-lg bg-brand px-3 py-1.5 text-sm font-medium text-brand-fg hover:bg-brand-hover disabled:opacity-50"
+        >
+          <Save size={14} strokeWidth={2} />
+          {pending ? "Saving…" : "Save"}
+        </button>
+        {saved && !dirty && (
+          <span className="text-xs text-text-muted">Saved.</span>
+        )}
+        {error && (
+          <span role="alert" className="text-xs text-accent">
+            {error}
+          </span>
+        )}
+      </div>
+    </section>
+  );
+}