feat(groups): a group becomes somewhere you can go

It was a heading over some rows: a name, a colour, and whether it was folded.
Enough to tidy a list, and not enough to be what people mean by it — "Arbejde" is
a body of work with context of its own, and documents that belong to all of it
rather than to any one project inside it.

`/gruppe/{id}` carries three things a heading cannot. The projects in it, each
with what is still open — open tasks, not all of them, because "12" beside a
project somebody finished last year is a number that means nothing. A description,
written in place and saved on blur, the same shape a task's description has. And
the files that govern the whole group: the contract, the drawing, the agreement
everything else rests on.

The name in the sidebar is a link to it and the chevron beside it still folds.
Two intentions, two targets — the single one they used to share could only ever
serve the smaller.

Migration 0012 rebuilds `attachments`, because its CHECK spelled out exactly which
parent columns may be set and SQLite cannot alter a CHECK. The rule is still
"exactly one parent", counted rather than written out as pairwise comparisons:
with three columns that is three clauses, with four it is six, and the version
that is wrong looks exactly like the one that is right.

A group's file is reachable by its owner and nobody else. A group belongs to one
person and is never shared, so `reachAttachment` asks about ownership rather than
a project role — both old callers resolved straight to a task, so a group's
attachment answered 404 for the person who uploaded it.

Three tests also stopped racing their own navigation. Creating a project navigates
and `goto` is a promise nobody was holding, so a quick add fired straight after
could land in the Inbox instead — intermittent, and it read as the quick add
having failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/migrations/0012_groups_are_places.sql

@@ -0,0 +1,53 @@
+-- A group becomes somewhere you can go, rather than only a heading.
+--
+-- Until now a project group was a label over some rows in the sidebar: a name, a
+-- colour, and whether it was folded. That is enough to tidy a list and not enough
+-- to be the thing people actually mean by it — "Arbejde" is a body of work with
+-- context of its own: what it is, and the documents that belong to all of it
+-- rather than to any one project inside it.
+--
+-- Two changes. A description on the group, and attachments that can hang on one.
+--
+-- The attachments table has to be rebuilt for the second, because its CHECK spells
+-- out exactly which parent columns may be set, and SQLite cannot alter a CHECK.
+-- Foreign keys go off for the length of it — see the marker below and the note in
+-- `applyMigration` — because dropping the old table with them on is a cascading
+-- delete of every attachment in the instance.
+--
+-- The rule stays "exactly one parent": an attachment belongs to a task, a comment
+-- or a group, never two. An attachment with two parents is one that gets deleted
+-- once and stays visible in the other place.
+--
+-- verdande:rebuild-tables
+
+ALTER TABLE project_groups ADD COLUMN description TEXT NOT NULL DEFAULT '';
+
+CREATE TABLE attachments_rebuilt (
+    id          TEXT PRIMARY KEY,
+    task_id     TEXT REFERENCES tasks (id) ON DELETE CASCADE,
+    comment_id  TEXT REFERENCES comments (id) ON DELETE CASCADE,
+    group_id    TEXT REFERENCES project_groups (id) ON DELETE CASCADE,
+    filename    TEXT NOT NULL,
+    mime_type   TEXT NOT NULL DEFAULT 'application/octet-stream',
+    size        INTEGER NOT NULL,
+    path        TEXT NOT NULL,
+    uploaded_by TEXT NOT NULL REFERENCES users (id) ON DELETE CASCADE,
+    created_at  INTEGER NOT NULL,
+    -- Exactly one parent, counted rather than spelled out as a chain of pairwise
+    -- comparisons: with three columns that is three clauses, with four it is six,
+    -- and the version that is wrong looks exactly like the version that is right.
+    CHECK ((task_id IS NOT NULL) + (comment_id IS NOT NULL) + (group_id IS NOT NULL) = 1)
+);
+
+INSERT INTO attachments_rebuilt
+    (id, task_id, comment_id, filename, mime_type, size, path, uploaded_by, created_at)
+SELECT id, task_id, comment_id, filename, mime_type, size, path, uploaded_by, created_at
+FROM attachments;
+
+DROP TABLE attachments;
+
+ALTER TABLE attachments_rebuilt RENAME TO attachments;
+
+CREATE INDEX idx_attachments_task ON attachments (task_id) WHERE task_id IS NOT NULL;
+CREATE INDEX idx_attachments_comment ON attachments (comment_id) WHERE comment_id IS NOT NULL;
+CREATE INDEX idx_attachments_group ON attachments (group_id) WHERE group_id IS NOT NULL;