feat: pre/post/error/finally lifecycle hooks and ordered backup execution (#108)

[muebau]

Closes #108

Summary

Adds support for user-defined lifecycle hooks and ordered backup execution,
as proposed in #108.

Hooks

Labels on any container define commands that run around the backup process:

stack-back.hooks.<stage>.<N>.cmd        # command to run
stack-back.hooks.<stage>.<N>.context    # service to exec into (optional)
stack-back.hooks.<stage>.<N>.on-error   # "abort" (default) | "continue"

Stages follow try/catch/finally semantics:

Stage	Runs when
pre	Before backup starts
post	Only if all pre hooks + backup succeeded
error	Only on failure
finally	Always, regardless of outcome

Context resolution: When context is omitted, the hook runs in the
container where the label is defined — backup-container hooks run locally,
target-container hooks run via docker exec. An explicit context overrides
this and execs into the named service.

Symmetric ordering: Pre hooks run backup-container first, then targets
(setup). Post/error/finally hooks reverse this: targets first, then
backup-container (teardown).

Ordered backup

Labels on the backup container control execution order:

stack-back.ordered: true
stack-back.order.1: "nextcloud-app"
stack-back.order.2: "nextcloud-database"

Containers not listed are appended after the ordered ones. Duplicates,
unknown services, and gaps in numbering are handled with warnings.

Lifecycle change

The backup lifecycle in start_backup_process() was restructured so that
stopped containers are always restarted before any post/error/finally
hooks execute. This ensures hooks can exec into previously-stopped services.

pre hooks → stop containers → backup volumes & DB dumps → restart containers →
post hooks (on success) / error hooks (on failure) → finally hooks (always)

Changed files

File	Change
hooks.py	New — hook parsing, collection, ordering, execution
cli.py	Lifecycle restructured with try/finally, hook integration, label forwarding
containers.py	Ordered backup via _parse_backup_order()
enums.py	LABEL_ORDERED, LABEL_ORDER_PREFIX constants
test_hooks.py	New — hook parsing, ordering, execution, context resolution
test_backup_lifecycle.py	New — operation sequencing with mocked hooks
test_end_to_end_lifecycle.py	New — full pipeline with real objects, I/O mocked