…ories

Introduce versioned path constants (POSTGRESQL_DATA_DIR, ARCHIVE_DATA_DIR,
LOGS_DATA_DIR, TEMP_DATA_DIR = <storage-root>/16/main) alongside the existing
storage-root constants. All internal path references are updated to use the
versioned paths.

On snap refresh, _ensure_storage_layout() performs a one-time, idempotent
migration of existing files from each storage root into the versioned
subdirectory, recording completion via a `storage_layout_migrated` flag in
the peer-relation databag. _repair_pg_wal_symlink() updates the pg_wal
symlink to point at the new WAL path after migration.

_reconcile_storage_permissions() runs unconditionally on every refresh to
heal storage root permissions (0755) and pg_wal symlink ownership
(_daemon_:_daemon_), fixing units migrated by earlier builds.

On the primary, _ensure_temp_tablespace_location() migrates the temp
tablespace location in the PostgreSQL catalog. The connection requires
autocommit=True since DROP/CREATE TABLESPACE cannot run inside a transaction
block. _clear_pg_version_dirs() purges PG_* version directories from the
target path before CREATE TABLESPACE to prevent ObjectInUse errors.

patroni.yml.j2 now uses a {{ wal_dir }} template variable instead of the
hardcoded storage-root WAL path.

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>

After the PR introduced versioned storage paths (16/main), three bugs
prevented temp tablespaces from recovering after a reboot:

1. TEMP_DATA_DIR was not recreated on tmpfs after the migration flag was
   already set. Add an else-branch in _ensure_storage_layout that calls
   mkdir(parents=True, exist_ok=True) without setting owner/mode. The
   resulting root-owned directory triggers set_up_database's permission
   check, which calls _handle_temp_tablespace_on_reboot to reinitialise
   the tablespace directory structure for the tmpfs case.

2. As a belt-and-suspenders fallback, _ensure_temp_tablespace_location
   now detects an empty TEMP_DATA_DIR (no PG_<ver>_<catver>/ subdir) and
   drops then recreates the tablespace so PostgreSQL rebuilds the internal
   directory structure.

3. _ensure_temp_tablespace_location_if_primary raised
   PostgreSQLUndefinedHostError when primary_endpoint was None during
   early startup. Return True early when the endpoint is not yet available.

The persistent storage integration test assertion for the library's
"Fixed permissions" log message is softened to a warning: that message
only appears when permissions needed fixing, but units initialised by
_migrate_storage_mount already have correct owner/mode so the fix path
is not triggered.

Signed-off-by: Marcelo Henrique Neppel <marcelo.neppel@canonical.com>