feat: Add opt-in article descriptions and keywords

[DamienSchreurs]

Summary

This follows up on #18 and builds on the work by @hermes-lol and @twaithalek-ai.

• add nullable articles.description and articles.keywords columns through the existing additive migration path
• keep description and keyword collection disabled unless explicitly enabled for scan
• keep the existing articles output unchanged unless the corresponding display flags are requested
• preserve gofeed as the primary RSS, Atom, and JSON Feed parser
• extract repeated XML keywords in a supplementary, failure-isolated pass only when keyword storage is enabled
• cap keyword-mode buffering at 10 MiB of decompressed feed data
• store the first meaningful description paragraph, with content fallback and a configurable Unicode-safe character limit
• prefer nearby Unicode whitespace or punctuation when truncating, with an exact character-boundary fallback for CJK and long tokens

Opt-in interfaces

Storage:

blogwatcher scan --store-descriptions
blogwatcher scan --store-keywords
blogwatcher scan --store-descriptions --description-max-chars 500

Environment equivalents:

BLOGWATCHER_STORE_DESCRIPTIONS=true
BLOGWATCHER_STORE_KEYWORDS=true
BLOGWATCHER_DESCRIPTION_MAX_CHARS=1000

Output:

blogwatcher articles --show-descriptions
blogwatcher articles --show-keywords

Flags override environment variables. Description storage defaults to a 1,000-character limit when enabled; 0 means unlimited. Existing rows are not backfilled, and already stored values are not removed when storage is later disabled.

Compatibility

• a default scan writes NULL for both optional fields
• blogwatcher articles without either display flag is byte-for-byte compatible with v0.0.3 in an isolated fixture comparison
• the same database was exercised old → candidate → old after metadata population; v0.0.3 output remained byte-for-byte identical
• old binaries safely ignore the additive nullable columns
• no production database, scheduler, or news-curation integration is changed here

Verification

• gofmt
• go vet ./...
• go test -count=1 ./...
• candidate build
• exact-limit and oversized keyword-buffer tests
• CJK, Unicode punctuation, distant-whitespace, and exact character-boundary tests
• isolated SQLite migration and PRAGMA quick_check
• isolated CLI output and configuration smoke tests
• v0.0.3 versus candidate default-output comparison on equivalent fixture databases

Closes no issue automatically; #18 is left open for the maintainer to manage.

[Hyaxia]

thanks @DamienSchreurs!
i've made small changes there (making sure the oversized feed wont crash items for example).