feat: add notification template registry (#352)

[MAN7A-afk]

Description:

Closes #352

Adds an on-chain registry for reusable notification templates so senders can reference a stored template instead of repeating identical notification data each time.

Changes:

types.rs — added NotificationTemplate struct (id, owner, name, content, created_at, updated_at: Option)
events.rs — added TemplateRegistered and TemplateUpdated events, both emitting template_id with owner, category, and priority as topics
errors.rs — added three new error variants (TemplateNotFound, TemplateNameTooLong, TemplateContentEmpty), starting at discriminant 31 to avoid colliding with existing duplicate discriminants already present in the file
template_registry_logic.rs (new) — implements:
register_template — auth-checks the creator, rejects duplicate IDs, validates name/content, stores the template, emits TemplateRegistered
update_template — auth-checks the caller, enforces owner-only updates, rejects missing IDs, validates input, emits TemplateUpdated
get_template — returns the stored template or TemplateNotFound
template_exists — pure view function, never reverts
lib.rs — wired the four entry points into the contract impl block and registered the new test module
template_registry_test.rs (new) — 14 tests covering successful registration, duplicate rejection, validation guards (empty content, name too long), owner-only update enforcement, missing-ID handling, existence checks, and event emission with correct topic/data assertions

Testing:
All new files pass static diagnostics (no syntax/type errors). I wasn't able to run cargo test locally in this environment (Rust/Cargo wasn't installed on this machine at the time), so I'm relying on CI to confirm the test suite passes — will address any failures CI surfaces.

Notes for reviewers:

errors.rs had pre-existing duplicate discriminants (28, 29, 30) unrelated to this change — I left those as-is and started new variants at 31 to avoid touching unrelated code, but flagging it here in case it should be cleaned up separately.

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