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/image_results.html

@@ -0,0 +1,30 @@
+{#
+  Web image picker. Each result is a button that POSTs its URL to `post_url`;
+  the server downloads and normalizes it, then returns the icon-status block
+  that `status_target` points at.
+
+  Thumbnails load straight from the search engine's CDN, so the browser talks
+  to it directly — `referrerpolicy` keeps ImpTune's own URL out of that request.
+#}
+{% if hits %}
+<p class="hint">
+  <span x-data x-text="$store.i18n.t('image_results_for')">Results for</span>
+  <span class="mono">{{ query }}</span>
+</p>
+<div class="image-grid">
+  {% for h in hits %}
+  <button type="button" class="image-option"
+          hx-post="{{ post_url }}"
+          hx-vals='{{ {"url": h.url} | tojson }}'
+          hx-target="{{ status_target }}"
+          hx-swap="innerHTML"
+          title="{{ h.title }} — {{ h.source }}">
+    <img src="{{ h.thumbnail }}" alt="" loading="lazy" referrerpolicy="no-referrer">
+    <span class="image-meta mono">{{ h.width }}&times;{{ h.height }}</span>
+  </button>
+  {% endfor %}
+</div>
+<p class="hint" x-data x-text="$store.i18n.t('image_pick_hint')">Pick one — it is downloaded and resized to 256 × 256 PNG.</p>
+{% else %}
+<p class="dim" x-data x-text="$store.i18n.t('no_image_results')">No results. Try another search term.</p>
+{% endif %}