Adds automatic session linking/identity stitching to link anonymous
browsing sessions with authenticated user sessions.
## Changes
### Database Schema
- Add `identity_link` table (PostgreSQL + ClickHouse) to store mappings
between visitor IDs and authenticated user IDs
- Add `visitor_id` field to `Session` model
- Add `visitor_id` column to ClickHouse `website_event` table
### Client Tracker
- Generate and persist `visitor_id` in localStorage
- Include `vid` in all tracking payloads
- Support opt-out via `data-identity-stitching="false"` attribute
### API
- Accept `vid` parameter in `/api/send` endpoint
- Auto-create identity links when `identify()` is called with both
visitor_id and distinct_id
- Store visitor_id in sessions and events
### Query Updates
- Update `getWebsiteStats` to deduplicate visitors by resolved identity
- Visitors who browse anonymously then log in are now counted as one user
## Usage
When a user logs in, call `umami.identify(userId)`. If identity stitching
is enabled (default), the tracker automatically links the anonymous
visitor_id to the authenticated userId. Stats queries then resolve
linked identities to accurately count unique visitors.
Resolves#3820
