click_count column exists but is never maintained

ShortURL.to_dict() (used by GET /api/urls/<slug> and the list endpoint) computes click_count via self.clicks.count() -- a SELECT COUNT(*) FROM click_events WHERE short_url_id = ? on every single metadata request. For a link with modest traffic this is fine; for a link that has gone viral with millions of clicks, every metadata/list request pays for a full count scan over a table that only grows. A denormalized counter, updated once per click instead of scanned once per read, is the standard fix -- and the column for it (click_count) already exists in the schema (migration a7d7833a3e8e), it's just not wired up to anything yet.

click_count = db.Column(db.Integer, nullable=False, default=0, server_default="0") exists on ShortURL, but nothing anywhere increments it -- _record_click inserts a ClickEvent row and commits, and to_dict() still computes the count the old way (self.clicks.count()), completely ignoring the new column.

1. pytest practice_tickets/tests/test_ticket_15_click_count_column.py -v
2. Or manually: click a short URL 3 times, then check ShortURL.click_count directly.
3. Observe: click_count is 0, not 3.

The interesting part of this ticket isn't adding += 1 -- it's doing the increment in a way that's safe under concurrent clicks on the same link. short_url.click_count += 1; db.session.commit() (read-modify-write in Python) has a race: two concurrent requests can both read the same starting value and both write back old_value + 1, losing one increment. The safe version is an atomic, single-statement DB-side increment (e.g. UPDATE short_urls SET click_count = click_count + 1 WHERE id = :id, which SQLAlchemy can express without a Python- side read at all).

Look at _record_click for where the ClickEvent is inserted and committed -- the increment belongs in the same function, same transaction. Search for how SQLAlchemy expresses an atomic column increment (hint: it is an UPDATE using the column itself in the SET expression, not a Python += on a loaded instance). Once increments are wired up and correct, swap to_dict()'s self.clicks.count() for self.click_count.