Define locusview to locuscompare2 DB access (read-only serving role + connection config)

[boxiangliu]

Create a least-privilege, read-only DB role for the public serving path; wire connection via env vars (.env). Depends on #1.

[boxiangliu]

Resolved: locusview connects to the shared locuscompare2 DB read-only via LOCUSCOMPARE2_DB_* env (see docs/how-to/connect-to-locuscompare2-database.md); verified working live (#25).