refactor: move stable store SQL to sqlc

[steipete]

Summary

• Add sqlc infrastructure for stable Discrawl store SQL: sqlc.yaml, flattened compile-time schema, checked-in generated internal/store/storedb, and a scripts/generate-sqlc.sh helper.
• Wire Store to generated typed queries and convert stable read/write paths for sync state, status/counts, channels, guilds, members, attachments/media metadata, message upserts/deletes, embedding jobs, and member profile reads.
• Keep runtime migrations/schema version, FTS rebuild/probes, semantic/vector search, report/share/user SQL, and dynamic filter builders handwritten.
• Add schema drift coverage plus member enrichment regression coverage caught during review.

Review Notes

• Runtime migrations remain authoritative; internal/store/sqlc/schema.sql is a sqlc-only flattened mirror of schema version 3.
• Transactional conversions use q.WithTx(tx).
• Generated code is checked in so normal builds do not require sqlc.
• Git snapshot/share paths remain covered by the existing share and CLI tests; this PR does not move dynamic share SQL into sqlc.

Verification

• ./scripts/generate-sqlc.sh
• ./scripts/generate-sqlc.sh && git diff --exit-code -- internal/store/storedb internal/store/sqlc
• GOWORK=off go test ./internal/store
• GOWORK=off go test ./...
• DISCRAWL_NO_AUTO_UPDATE=1 GOWORK=off go run ./cmd/discrawl doctor
• DISCRAWL_NO_AUTO_UPDATE=1 GOWORK=off go run ./cmd/discrawl status --json
• DISCRAWL_NO_AUTO_UPDATE=1 GOWORK=off go run ./cmd/discrawl search --limit 5 test
• DISCRAWL_NO_AUTO_UPDATE=1 GOWORK=off go run ./cmd/discrawl sync --source wiretap
• Codex review: /Users/steipete/Projects/agent-scripts/skills/codex-review/scripts/codex-review --full-access --mode branch --parallel-tests "GOWORK=off go test ./internal/store"