feat(cli): --author and --since filters on buzz messages search

[tlongwell-block]

Implements the buzz-feature-requests ask (Aaron): query all messages by a specific user across channels without an N-channel sweep.

What

buzz messages search gains two flags:

• --author <pubkey|npub|display-name> — filter by author. Hex and npub are used directly; a display name is resolved via NIP-50 kind:0 search with an exact case-insensitive match on display_name/name. Zero matches or ambiguity (e.g. 100 users named "Fizz") is a clear usage error rather than a silent wrong-author query.
• --since <unix-ts> — passes through to the relay filter.

--query becomes optional; at least one of --query/--author is required. All flags combine.

# All of Aaron's messages today, across channels
buzz messages search --author Aaron --since 1783497600

# Aaron's messages mentioning "checkout"
buzz messages search --author Aaron --query checkout --limit 20

No relay changes

Both relay paths already honor the standard nostr authors filter field:

• --author + --query → HTTP bridge FTS path (handle_bridge_search) pushes authors into buzz_search::SearchQuery (bridge.rs).
• --author alone → general query path (build_event_query_from_filter) uses the authors IN-list pushdown in buzz-db and post-filters channel access as usual.

Author-only results are sorted newest-first client-side (the FTS path returns relevance order; a pure author/time query has none).

Verification

• cargo test -p buzz-cli — 139 pass, incl. 4 new unit tests pinning the name-resolution semantics (exact-not-substring, case-insensitive, ambiguity, dedup of duplicate kind:0 rows, malformed content).
• Clippy clean, cargo fmt applied.
• Live e2e against the staging relay: author-only (hex/npub/name), author+query, --since window bounds respected, and error paths (no flags, unknown name, ambiguous name with capped candidate listing, invalid npub) all verified.

The nice-to-haves from the request (--channel scope, --mentions) are deliberately left out per the "minimal ask" framing — both relay paths already support them if wanted later.