feat: driver rename, driver icons, web image + driver search

Driver rename and icons: `Driver.display_name` plus a `DriverIcon` table, both
global/shared like the `Driver` row they hang off, so a rename or an icon is
what every Owner sees. The rename/icon dialog keeps its forms as siblings
(nested forms are invalid HTML) and the icon routes return an `hx-swap-oob`
thumbnail refresh rather than re-rendering the table, which would tear the open
`<dialog>` out of the DOM.

Web image picker: `GET /web/images` renders a pickable grid for a printer or a
driver icon, with the search term prefilled from the entity name and editable.
Picking one downloads it server-side and normalizes it.

Driver download search: `GET /web/drivers` searches for a vendor-wide driver
(the term is rewritten into the vendor's real product name for 15 brands) or for
the exact model as typed. Links only — nothing is downloaded, and the fragment
says the results are unvetted.

Icon uploads no longer reject off-size or non-PNG files: `normalize_icon()`
letterboxes any decodable raster into a 256x256 PNG. An already-exact 256x256
PNG is returned byte-identical, because icon storage is content-addressed and
re-encoding would move the file on every save.

`fetch_image()` makes the request from the server, so `assert_fetchable()`
refuses any URL resolving to a private, loopback, or link-local address, and
re-runs on every redirect. ImpTune sits on the same LAN as the printers it
configures; an unguarded fetcher would be a port scanner for anyone who can
reach the UI.

DuckDuckGo is scraped, not called through an API — no key needed, but fragile,
so both search functions swallow parse failures and return [] instead of 500ing
a page. `WEB_SEARCH=false` disables every outbound request and hides the
controls, for air-gapped installs.

Also: one shared `Jinja2Templates` in `templating.py` instead of five per-router
instances, so a template global is declared once; `_add_missing_columns()` in
`database.py` adds new nullable columns to a pre-existing table, which
`create_tables(safe=True)` skips; `db_env` in test_db.py now closes its
connection on teardown, or the next test's ORM writes land in the previous
test's DB file.

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

imptune/templates/partials/driver_search_results.html

@@ -0,0 +1,29 @@
+{#
+  Driver download search results — links only. ImpTune never downloads a driver
+  for you: these are raw web results, and the warning below says so plainly.
+#}
+{% if hits %}
+<p class="hint">
+  <span x-data x-text="$store.i18n.t('searched_for')">Searched for</span>
+  <span class="mono">{{ query }}</span>
+</p>
+
+<div class="notice warn">
+  <p x-data x-text="$store.i18n.t('driver_search_warning')">
+    Unfiltered web results, not a vetted list. Download drivers only from the
+    manufacturer's own site, then upload the ZIP above.
+  </p>
+</div>
+
+<ul class="link-list">
+  {% for h in hits %}
+  <li>
+    <a href="{{ h.url }}" target="_blank" rel="noopener noreferrer nofollow">{{ h.title }}</a>
+    <span class="cell-sub mono">{{ h.host }}</span>
+    {% if h.snippet %}<p class="dim">{{ h.snippet }}</p>{% endif %}
+  </li>
+  {% endfor %}
+</ul>
+{% else %}
+<p class="dim" x-data x-text="$store.i18n.t('no_driver_results')">No results. Try the exact model name instead.</p>
+{% endif %}