feat(hub): group names unique per owner, shown as name@owner

A group's identity stays its UUID. What changes is that "the name is
unique" — until now an unenforced expectation — becomes real, scoped to the
owner account, and the owner's username is surfaced so two groups called
"photos" on different nodes can be told apart.

Hub:
- `groups` gains a functional unique index `uq_groups_owner_name` on
  `(admin_id, lower(name))` (model + migration c3d4e5f6a7b8). The migration
  pre-flights: if the data already clashes it aborts and lists the
  offending (admin_id, name) pairs rather than renaming anyone's group.
  meshbay.org checked clean.
- `create_group` trims the name, rejects blank (422) and an owner-scoped
  case-insensitive clash (409), with an IntegrityError backstop for the
  race, and returns `owner_username`.
- `owner_username` added to `/v1/groups/mine`, `GET /v1/groups` (local
  rows), `POST /v1/groups/{id}/join`, `GET /v1/admin/groups`.

SPA:
- new `static/group-name.js` — `<GroupName name owner [inline]>` renders the
  name with the `@owner` handle on a smaller grey line under it.
- used in the sidebar, the group-page header, Explore cards, the Admin
  groups table, and cross-group Search (via a widened `cacheGroupIndex`
  carrying the owner). Delete/leave confirmations show `name@owner` inline.
- federated Explore rows show `@<source_hub>` instead of an account.

Design record and the locked decisions: ~/next/groupnames.md (out of repo).
MNP unchanged. Tests: test_group_name_unique.py, test_group_name_migration.py.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_018gKJ85aZyvEwarXMFzFEwi

2 files changed, 54 insertions, 2 deletions

diff --git a/packages/meshbay-hub/src/meshbay_hub/db/migrations/versions/c3d4e5f6a7b8_group_name_unique_per_owner.py b/packages/meshbay-hub/src/meshbay_hub/db/migrations/versions/c3d4e5f6a7b8_group_name_unique_per_owner.py
new file mode 100644
index 0000000..6fc5b73
--- /dev/null
+++ b/packages/meshbay-hub/src/meshbay_hub/db/migrations/versions/c3d4e5f6a7b8_group_name_unique_per_owner.py
@@ -0,0 +1,45 @@
+"""group_name_unique_per_owner
+
+Revision ID: c3d4e5f6a7b8
+Revises: b1c2d3e4f5a6
+Create Date: 2026-08-28 14:00:00.000000
+
+One group name per owner, case-insensitively. The group's identity is still
+its UUID — this only makes `name@owner` a dependable handle.
+
+Pre-flight: abort if the data already violates it, with the offending
+(admin_id, name) pairs listed, rather than silently renaming anyone's group.
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+revision: str = 'c3d4e5f6a7b8'
+down_revision: Union[str, Sequence[str], None] = 'b1c2d3e4f5a6'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    bind = op.get_bind()
+    dupes = bind.execute(sa.text(
+        "SELECT admin_id, lower(name) AS n, count(*) AS c "
+        "FROM groups GROUP BY admin_id, lower(name) HAVING count(*) > 1"
+    )).fetchall()
+    if dupes:
+        listed = ", ".join(f"{r.admin_id[:8]}/{r.n!r}×{r.c}" for r in dupes)
+        raise RuntimeError(
+            "Cannot add uq_groups_owner_name: an owner already has two groups "
+            f"with the same name (case-insensitively): {listed}. "
+            "Rename or delete one of each pair, then re-run the migration."
+        )
+    op.create_index(
+        "uq_groups_owner_name", "groups",
+        ["admin_id", sa.text("lower(name)")], unique=True,
+    )
+
+
+def downgrade() -> None:
+    op.drop_index("uq_groups_owner_name", table_name="groups")
diff --git a/packages/meshbay-hub/src/meshbay_hub/db/models.py b/packages/meshbay-hub/src/meshbay_hub/db/models.py
index 0c337c1..dbabfc2 100644
--- a/packages/meshbay-hub/src/meshbay_hub/db/models.py
+++ b/packages/meshbay-hub/src/meshbay_hub/db/models.py
@@ -15,7 +15,7 @@ from datetime import datetime, timezone
 
 from sqlalchemy import (
     Boolean, DateTime, ForeignKey, Index, Integer,
-    String, Text, UniqueConstraint,
+    String, Text, UniqueConstraint, text,
 )
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
 
@@ -109,7 +109,14 @@ class Group(Base):
 
     members:     Mapped[list["GroupMember"]] = relationship(back_populates="group")
 
-    __table_args__ = (Index("ix_groups_name", "name"),)
+    __table_args__ = (
+        Index("ix_groups_name", "name"),
+        # One name per owner, case-insensitively. The group's identity stays its
+        # UUID; this is what makes `name@owner` a handle a human can rely on
+        # (two different owners may still both have a "photos"). Enforced in the
+        # DB so a race cannot slip a second one past the check in create_group.
+        Index("uq_groups_owner_name", "admin_id", text("lower(name)"), unique=True),
+    )
 
 
 class GroupMember(Base):