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/drivers.html

@@ -38,6 +38,44 @@
   </div>
 </section>
 
+{% if web_search_enabled() %}
+<section class="card">
+  <div class="card-head">
+    {{ ico.i('search') }}
+    <div>
+      <h2 x-data x-text="$store.i18n.t('find_driver_online')">Find a driver online</h2>
+      <p class="sub" x-data x-text="$store.i18n.t('find_driver_intro')">Search for a vendor-wide driver, or for an exact model. ImpTune only shows links — you download the ZIP and upload it above.</p>
+    </div>
+  </div>
+  <div class="card-body">
+    {# Two submit buttons, one form: the clicked button's `mode` value is what
+       gets sent, so "generic" and "exact" are one round-trip apart. #}
+    <form hx-get="/web/drivers"
+          hx-target="#driver-search-results"
+          hx-swap="innerHTML"
+          hx-indicator="#driver-search-spinner">
+      <label>
+        <span class="label-text" x-data x-text="$store.i18n.t('driver_search_label')">Brand or printer model</span>
+        <input type="search" name="q" list="driver-brands" placeholder="HP LaserJet M404">
+      </label>
+      <datalist id="driver-brands">
+        {% for brand in brands %}<option value="{{ brand }}">{% endfor %}
+      </datalist>
+      <div class="btn-row">
+        <button type="submit" class="btn" name="mode" value="generic" x-data>
+          {{ ico.i('search', 14) }}<span x-text="$store.i18n.t('search_generic_btn')">Find generic driver</span>
+        </button>
+        <button type="submit" class="btn ghost" name="mode" value="exact" x-data
+                x-text="$store.i18n.t('search_exact_btn')">Search exact model</button>
+        <span id="driver-search-spinner" class="htmx-indicator" aria-busy="true"
+              x-data x-text="$store.i18n.t('searching')">Searching…</span>
+      </div>
+    </form>
+    <div id="driver-search-results"></div>
+  </div>
+</section>
+{% endif %}
+
 <h2 class="eyebrow" style="margin-bottom:.6rem" x-data x-text="$store.i18n.t('driver_library')">Driver library</h2>
 {% include "partials/driver_list.html" %}
 {% endblock %}