fix: handle email-only feeds and HTML responses in feed fetcher

[bokelley]

Summary

• Add RSS content validation to detect when feeds return HTML instead of XML (e.g., when sites disable their RSS feeds and redirect to homepage)
• Allow NULL feed_url for email-only feeds in database schema
• Update getFeedsToFetch() to exclude email-only feeds (feed_url IS NULL)
• Migrate existing email:// placeholder URLs to NULL

Problem

Tipsheet.ai (and potentially other sites) have disabled their RSS feeds. When the fetcher tried to parse the redirected HTML as XML, it failed with a cryptic error: Invalid character in entity name Line: 43 Column: 1084 Char: &

Additionally, the database schema had a NOT NULL constraint on feed_url, preventing email-only feeds from being created through the UI (which sends feed_url: null).

Solution

1. Pre-fetch validation: Fetch content before parsing to validate it's actually RSS/XML, not HTML
2. Clear error messages: Show "Feed URL returned HTML instead of RSS/XML (feed may be disabled)" instead of cryptic XML parsing errors
3. Schema migration: Allow NULL feed_url with partial unique index for non-null URLs
4. Query update: Exclude email-only feeds from RSS fetch queries

Test plan

• Typecheck passes
• All unit tests pass
• End-to-end tested: Created email-only feed through UI successfully
• Verified database shows feed_url = NULL for email-only feeds

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