feat(core): add UserLog model and REST resource for browser error logging

[Terracosmos]

Context

Backend part of ligoj/plugin-ui#34 (record browser-side errors and expose them in
an admin "User logs" view). This PR adds the data model and the REST resource in
plugin-core, as the issue specifies. The front-end capture and the admin UI come
in separate PRs (host + plugin-ui).

What it does

• UserLog entity → table LIGOJ_USER_LOG, created automatically by Hibernate
  (jpa.hbm2ddl=update), so no SQL migration is shipped.
• UserLogResource (/user-log):
  • POST records one error; the server stamps the login (SecurityHelper.getLogin())
    and the date, so a user can only log on its own behalf. Body: { message, url }.
  • GET returns the logs paginated, newest first, with an optional from/to
    date range (epoch ms). Restricted to administrators.

Design decisions (and why)

• New dedicated entity rather than reusing Event: Event is node/subscription
  scoped; this log is user-scoped with different columns, so a clean entity is simpler.
• Column user mapped to user_login: USER is a reserved keyword on several
  databases (H2/HSQLDB/PostgreSQL) and breaks the generated DDL otherwise.
• message capped at 2000 chars (truncated server-side), url at 512 and stored
  path-only (no domain) — consistent with the bug-report dialog (plugin-ui#33).
• Date filter uses explicit bounds (EPOCH .. 9999) instead of :param IS NULL OR …:
  the latter fails on HSQLDB (untyped NULL parameter); the bounds are portable
  across all databases and semantically equivalent to "no bound".
• Sorting: id and date are passed as case-sensitive columns so pagination does
  not wrap them in lower() (invalid on numeric/temporal types).

Security

• GET is admin-only by two mechanisms: Ligoj's RBAC filter is default-deny (no
  USER authorization pattern covers /user-log, so only ADMIN .* matches),
  AND an explicit @PreAuthorize("hasAuthority('ADMIN')") on the method.
• POST is meant to be open to any authenticated user. That requires ONE line in
  the host authorization seed (ligoj/app-api/.../system-authorization.csv):
  POST;^rest/user-log.*;API;USER. That change lives in the host repo and ships
  with the front-end capture PR — NOT here.
• Note: an open POST is a potential spam vector; the front-end will throttle/dedupe,
  and a server-side rate limit could be added later if needed.

Integration note (for the release process)

plugin-core is consumed by the host via a pinned api.version. Going live therefore
needs a plugin-core release and a host api.version bump — a release-management step
on your side. Locally this was validated by bumping api.version to the SNAPSHOT
(not committed): table auto-created, POST → 204, GET → 200 returning the row.

What's not in this PR

• Front-end error capture (host) + the POST authorization line.
• The admin "User logs" UI (plugin-ui).

Tests

mvn -pl plugin-core -am install → BUILD SUCCESS, 370 tests green incl. 5 new
UserLogResourceTest (create, truncation, date filter, sort date desc).

Feedback very welcome — happy to adjust the contract (paths, payload, naming) to
match how you'd like the model/resource shaped in plugin-core.