Add pagination to GET /api/contacts

GET /api/contacts currently returns every contact in one response, always. That's fine for a demo with a handful of rows, but for any real account with hundreds or thousands of contacts, every list request pulls the entire table and serializes all of it -- slow responses, wasted bandwidth, and a client (mobile app, web UI) that has to hold the whole dataset in memory just to show the first screen. This is the kind of gap that's invisible in a small dev database and becomes a P1 "the contacts page is timing out" incident the day a customer imports a few thousand contacts.

list_contacts() in app/blueprints/contacts/routes.py already reads ?page= and ?per_page= from the query string and calls paginate(query, page, per_page) (imported from the new app/pagination.py module) -- but paginate() is currently just a stub that raises NotImplementedError. Calling GET /api/contacts without page/per_page is unaffected; calling it with page/per_page currently 500s.

for i in 1 2 3 4 5; do curl -s -X POST http://127.0.0.1:5000/api/contacts -H 'Content-Type: application/json' -d "{"name":"Person $i"}" > /dev/null done curl -s "http://127.0.0.1:5000/api/contacts?page=1&per_page=2"

-> HTTP 500 {"error":"internal_server_error","details":"an unexpected error occurred"}

Expected: HTTP 200 with a 2-item page and total=5

Pagination has to interact correctly with the already-filtered query -- list_contacts() builds up query with optional search/group filters before your code runs, so paginate() needs to slice that query (ordering already applied), not re-query contacts from scratch. Getting total right also means counting the filtered result set, not just len(items) on the current page -- a common pagination mistake.

Look at app/pagination.py::paginate(query, page, per_page) and how it's invoked from list_contacts(). SQLAlchemy's Query object supports .count(), .limit(), and .offset() -- think about what offset corresponds to page page with per_page items per page (page 1 should NOT skip any rows).