feat(notify): configurable plain/html summary formatting

Rework the Pushover/Apprise summary so it reads as a report instead of a
dump of counters.

- Extract build_summary() as the single source of truth for both notifiers,
  removing the duplicated body string and the stray leading indentation
  caused by backslash line continuations.
- Add a 'format' option: 'plain' (aligned columns) or 'html' (bold labels,
  sets html=1 for Pushover). Apprise always receives the plain body.
- Add 'use_emoji' for emoji-prefixed rows.
- Title now states the outcome ('7 torrents removed') instead of
  '--- qbit-maid summary ---'.
- Say 'Would remove' when delete_torrents is false, so a dry run is never
  mistaken for a real deletion.
- Format runtime as 12s / 1m 04s instead of a timedelta with microseconds.
- Add include_hostname, notify_on_empty, priority, sound, retry and expire.
- Log notification failures instead of letting them abort the run.
- Cover the new formatting with unit tests; document the options in the
  README and config.toml.example.

config.toml.example

@@ -23,8 +23,19 @@ delete_torrents = false
 
 [pushover]
 use_pushover = false
-po_key = "<key>>"
-po_token = "<token>>"
+po_key = "<key>"
+po_token = "<token>"
+# Notification formatting
+# format: "plain" (aligned text) or "html" (bold labels, Pushover HTML)
+format = "plain"
+use_emoji = false
+include_hostname = false
+notify_on_empty = true
+# Optional: -2..2. Priority 2 also needs retry/expire (defaults shown).
+# priority = 0
+# sound = "pushover"
+# retry = 60
+# expire = 3600
 
 [apprise]
 use_apprise = false
@@ -51,4 +62,4 @@ save = "save"
 
 [healthcheck]
 use_healthcheck = true
-healthcheck_url = "https://example.com/ping/<uuid>>"
+healthcheck_url = "https://example.com/ping/<uuid>>"