Put a keyword on a photograph, not only search for one

The catalog has been able to *find* by keyword since v1 — query.rs joins the
keywords table, matches it exactly, and substring-matches it for free text — and
nothing anywhere could ever put a word there. A user could filter to a keyword
they had no way to apply.

This is the missing half: create, rename, delete, list, assign, unassign, and
the two reads a panel needs. Bulk-only for assignment, because keywording a
selection is the common case rather than the exception — the photographer picks
out the frames with the puffin in them and applies "puffin" once, in one
transaction.

Schema v6 adds `keyword_terms`, and deliberately does *not* touch the v1 join.
The assignment keeps the word as text because the catalog is a rebuildable index
and the durable copies of that fact — the sidecar, XMP dc:subject — both carry a
string; a foreign key would mean a catalog rebuilt from sidecars had to invent
identity rows before it could record anything, and would break the query path
that already works. So the text is the fact, and the new table is only the
identity a rename and a deletion can be keyed on.

`keyword_terms.name` carries no unique index, which looks like an oversight and
is not: two devices that each type "Iceland" are both right until they meet, and
a constraint would abort the merge at that moment. Uniqueness is converged upon
instead — create resolves an existing name, fuse_duplicates collapses a
cross-device pair onto the smaller uuid.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/dr-catalog/src/collections.rs

@@ -682,7 +682,7 @@ fn require_exists(conn: &Connection, id: CollectionId) -> Result<(), CatalogErro
 /// same reasoning as the connector's date parsing. Version 4 layout, seeded
 /// from the OS via `getrandom` through `rusqlite`'s existing dependency-free
 /// path — see below.
-fn new_uuid() -> String {
+pub(crate) fn new_uuid() -> String {
     let b = random_bytes();
     // Version 4, variant 1, per RFC 4122 §4.4.
     let v6 = (b[6] & 0x0F) | 0x40;