Upgrade mautrix-telegram (v0.15.3 -> v0.2604.0) (bridgev2) and adapt configuration

Matches the earlier Python -> Go rewrites of the other mautrix-* bridges.

Related to:
- https://github.com/mautrix/telegram/releases/tag/v0.2604.0
- https://mau.fi/blog/2026-04-mautrix-release/

The bridge is now a Go binary with upstream-handled automatic database and
config migration on first start, so in-place upgrades on Postgres should
Just Work for users on the defaults. The lottieconverter sidecar container
is gone (bundled upstream), and the public web-based login endpoint is
gone (login happens inside Matrix now).

Upstream v0.2604.0 has a known bug in the legacy SQLite migration that
can corrupt data. The role detects legacy Python-bridge SQLite databases
(via the `telethon_sessions` table signature) and refuses to upgrade,
pointing users to switch to Postgres (playbook-managed pgloader migration)
or wait for the next upstream release. The guard is isolated in its own
`validate_config_sqlite_legacy_migration_bug.yml` so it can be deleted
cleanly once upstream fixes the bug.

Removed variables (all caught by the deprecation check in
`validate_config.yml` with actionable rename/removal hints): the entire
`_hostname` / `_path_prefix` / `_scheme` / `_public_endpoint` /
`_appservice_public_*` / `_container_labels_public_endpoint_*` /
`_container_http_host_bind_port` family (web login endpoint is gone);
`_bot_token` (old-style relaybot is gone, use the common bridgev2 relay
mode); `_filter_mode` (dropped upstream); `_bridge_login_shared_secret_map*`
(use Appservice Double Puppet); `_username_template`, `_alias_template`,
`_displayname_template` (templates moved under `network:`, new Go-template
syntax, exposed via `_network_displayname_template`); all
`_lottieconverter_*` variables; `_appservice_database` (renamed to
`_appservice_database_uri`).

Added playbook-time validation that catches legacy permission values
(`relaybot`, `puppeting`, `full`) in the fully-merged config (so overrides
via `matrix_mautrix_telegram_configuration_extension_yaml` are caught too),
with a mapping hint in the error message.

Other notes:

- The legacy sqlite->postgres relocation of `{base_path}/mautrix-telegram.db`
  to `{data_path}/mautrix-telegram.db` now happens BEFORE the pgloader
  migration step, so users who flip to Postgres as part of this upgrade
  get their data imported correctly.
- The Ketesa managed-user regex for the telegram namespace is updated to
  match both regular IDs and the new `channel-<id>` form used by bridgev2.
- `matrix_playbook_migration_expected_version` bumped to v2026.04.24.0,
  with a new breaking-change entry pointing at the CHANGELOG section.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CHANGELOG.md

@@ -1,3 +1,20 @@
+# 2026-04-24
+
+## (BC Break) mautrix-telegram has been rewritten in Go (bridgev2)
+
+The [mautrix-telegram](./docs/configuring-playbook-bridge-mautrix-telegram.md) bridge has been [rewritten in Go](https://mau.fi/blog/2026-04-mautrix-release/) on top of the [bridgev2](https://docs.mau.fi/bridges/go/) architecture. See the [upstream v26.04 release notes](https://github.com/mautrix/telegram/releases/tag/v0.2604.0) for what changed in the bridge itself (shared-portal behavior, management-room state, new features, etc.).
+
+**Most users won't have to do anything.** If you use the playbook's integrated Postgres (the default) and haven't customized telegram-bridge variables beyond `matrix_mautrix_telegram_api_id` and `matrix_mautrix_telegram_api_hash`, just re-run the playbook; the bridge will migrate itself on first start. Taking a backup beforehand is still a good idea.
+
+⚠️ **SQLite users: do not upgrade yet.** Upstream v0.2604.0 has a [known bug in the legacy SQLite migration](https://github.com/mautrix/telegram/releases/tag/v0.2604.0) that can corrupt your data. The playbook detects this case and will refuse to proceed. Either switch to Postgres first (set `matrix_mautrix_telegram_database_engine: postgres`; the playbook handles the pgloader migration), or wait for the next upstream release.
+
+Playbook-specific things to know. The playbook will fail loudly if you're affected:
+
+- Many `matrix_mautrix_telegram_*` variables have been **removed** (web-login endpoint, lottieconverter, username/alias/displayname templates, filter-mode, bot-token relaybot, Shared-Secret-Auth map). The deprecation check will tell you exactly what to rename or drop when you run the playbook.
+- **Old-style relaybot users** (`matrix_mautrix_telegram_bot_token`): switch to the common [mautrix bridge relay mode](./docs/configuring-playbook-bridge-mautrix-bridges.md#enable-relay-mode-optional) via `matrix_mautrix_telegram_bridge_relay_enabled: true`.
+- **Shared-Secret-Auth double-puppeting users**: switch to [Appservice Double Puppet](./docs/configuring-playbook-appservice-double-puppet.md); the playbook wires it up automatically.
+- **Custom `matrix_mautrix_telegram_bridge_permissions`**: map `relaybot` to `relay`, `puppeting` to `user`, `full` to `user`. Validated at playbook time.
+
 # 2026-04-03
 
 ## (BC Break) Synapse Admin (fork by etke.cc) is now Ketesa