fix(migrations): use SHA-256 instead of MD5 for the enclosures unique index

PostgreSQL 18 disables MD5 when running in FIPS mode, which made
md5(url) unusable. This broke enclosure creation with:

    store: unable to create enclosure: pq: could not compute MD5
    hash: unsupported (XX000)

Replace md5(url) with encode(sha256(url::bytea), 'hex') everywhere:

- The historical migrations that created the enclosures index are
  changed to sha256 so fresh installs no longer fail while replaying
  them on FIPS-mode PostgreSQL 18.
- A new migration rebuilds enclosures_user_entry_url_unique_idx with
  sha256 to convert existing installs.
- The ON CONFLICT clause in createEnclosure is updated to match the
  new expression index.

According to `openssl speed -bytes 256 md5 sha256`, this is a performance
improvement as well :D

Finally, the PostgreSQL minimum version was bumped from 9.5 to 11, the lowest
version to support SHA256.

Fixes #4350