sequences.excludeFilter is stored but never applied when sending #41

sequences.excludeFilter (apps/api/src/db/schema.ts:1168) is stored and accepted by the API and MCP (mcp/tools/schemas.ts:141), but the automation code never reads it. A user who sets an exclusion segment ("everyone except customers") still emails the excluded contacts.

Fix: apply excludeFilter when resolving the audience for broadcasts and sequences (as AND NOT (...) in the segment SQL), and add tests. If exclusion isn't meant to be supported yet, reject it at the API instead of accepting it silently. Exclusion will also be the natural way to leave dormant contacts out (#29).