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/printer_detail.html

@@ -192,16 +192,40 @@
               hx-target="#icon-status" hx-swap="innerHTML"
               hx-encoding="multipart/form-data">
           <div class="uploader">
-            <input type="file" name="file" accept="image/png" required
+            <input type="file" name="file" accept="image/*" required
                    x-data :aria-label="$store.i18n.t('upload_icon')">
             <div class="btn-row">
               <button type="submit" class="btn ghost sm" x-data>
                 {{ ico.i('upload', 13) }}<span x-text="$store.i18n.t('{{ 'replace_icon' if has_icon else 'upload_icon' }}')">Upload icon</span>
               </button>
             </div>
-            <p class="hint" x-data x-text="$store.i18n.t('hint_icon')">PNG, exactly 256 × 256, 750 KB max.</p>
+            <p class="hint" x-data x-text="$store.i18n.t('hint_any_image')">Any image — resized to 256 × 256 PNG. 750 KB max.</p>
           </div>
         </form>
+
+        {% if web_search_enabled() %}
+        <div class="web-picker">
+          <form hx-get="/web/images"
+                hx-target="#printer-image-results"
+                hx-swap="innerHTML"
+                hx-indicator="#printer-image-spinner">
+            <input type="hidden" name="target" value="printer">
+            <input type="hidden" name="id" value="{{ printer.id }}">
+            <label>
+              <span class="label-text" x-data x-text="$store.i18n.t('search_web_image')">Search the web for an image</span>
+              <input type="search" name="q" value="{{ image_query }}">
+            </label>
+            <div class="btn-row">
+              <button type="submit" class="btn ghost sm" x-data>
+                {{ ico.i('search', 13) }}<span x-text="$store.i18n.t('search_btn')">Search</span>
+              </button>
+              <span id="printer-image-spinner" class="htmx-indicator" aria-busy="true"
+                    x-data x-text="$store.i18n.t('searching')">Searching…</span>
+            </div>
+          </form>
+          <div id="printer-image-results"></div>
+        </div>
+        {% endif %}
       </div>
     </section>
   </div>