feat(ai): add a Reset AI Environment admin feature for the upgrade gap (#459)

Uninstalling a bundle only deletes its downloaded model weights, never the
shared venv's site-packages, so self-hosters who already hit an AI bundle
conflict (e.g. the scipy ABI strand) have no clean self-service path via
uninstall+reinstall: reinstalling just overlays corrected files on top of
stale ones. Adds POST /api/v1/admin/features/reset, which wipes
/data/ai/{venv,models,pip-cache}, resets installed.json, and reseeds a real
working venv from the image's baked /opt/venv (extracted docker/reseed-ai-venv.sh,
now shared with entrypoint.sh's existing base-venv-upgrade bootstrap instead
of duplicating that logic) -- leaving an empty venv directory here would
make the very next install fail with "spawn .../python3 ENOENT", caught by
testing this live rather than assuming it. Ships with a matching Settings UI
section (inline confirm, same pattern as per-bundle uninstall) and strings
across all 21 locales.

Verified against a real snapotter/snapotter:1.17.2 image migrated to 2.0.0,
with real multi-GB bundles installed (background-removal + OCR): confirmed
the migrated instance's inherited python3.11 venv (2.0.0 itself uses 3.12)
still imports the fixed scipy/numpy/paddleocr correctly, then reset + real
reinstall + actual tool execution (remove-background, verified output image)
all worked end-to-end.

docker/Dockerfile

@@ -583,10 +583,12 @@ COPY docker/entrypoint.sh /usr/local/bin/entrypoint.sh
 COPY docker/entrypoint-lib.sh /usr/local/bin/entrypoint-lib.sh
 COPY docker/embedded-lib.sh /usr/local/bin/embedded-lib.sh
 COPY docker/embedded/postgres-bootstrap.sh /usr/local/bin/embedded-postgres-bootstrap.sh
+COPY docker/reseed-ai-venv.sh /usr/local/bin/reseed-ai-venv.sh
 COPY docker/wait-for-postgres.mjs /app/docker/wait-for-postgres.mjs
 # s6-overlay reads its service tree from /etc/s6-overlay/s6-rc.d (embedded mode)
 COPY docker/s6/s6-rc.d /etc/s6-overlay/s6-rc.d
 RUN chmod +x /usr/local/bin/entrypoint.sh /usr/local/bin/embedded-postgres-bootstrap.sh \
+    /usr/local/bin/reseed-ai-venv.sh \
     && chmod +x /etc/s6-overlay/s6-rc.d/postgres/run /etc/s6-overlay/s6-rc.d/redis/run \
                 /etc/s6-overlay/s6-rc.d/snapotter/run \
                 /etc/s6-overlay/s6-rc.d/postgres-init/up /etc/s6-overlay/s6-rc.d/postgres-ready/up \