Normalize job-state vocabulary across plugins (queue_info / psij → batch_system.STATE_*)

[andre-merzky]

Follow-up split out of #14 (error propagation & status normalization) — the status-vocabulary half, which was explicitly scoped OUT of PR #91.

Problem

There is one shared job-state vocabulary — `batch_system.STATE_*` (`PENDING/RUNNING/DONE/FAILED/CANCELLED/HELD/UNKNOWN` + `TERMINAL_STATES`) — but plugins don't use it consistently:

• `queue_info_slurm.py` / `queue_info_pbs.py` emit raw scheduler state strings in their job listings.
• `plugin_psij.py` normalizes via its own `_normalize_state`, not the shared constants.
• The Explorer's `jobStateBadge` (added in Fix #40: surface more quinfo job info + consistent state formatting #90) keys off the raw strings.

So cross-plugin/job-state logic and the UI have no single vocabulary.

Proposed approach (non-destructive)

• Add a shared `normalize_job_state(raw) -> STATE_*` in `batch_system`.
• `queue_info` adds a `state_norm` field alongside the raw `state` (so Fix #40: surface more quinfo job info + consistent state formatting #90's raw-state badge map keeps working); re-point `psij._normalize_state` at the shared normalizer.
• Unit-test the mapping for SLURM + PBS raw states.

Out of scope: the HTTP-error envelope (done in #91).