sql: rewrite scid string literals to use scid() for index support

[vincenzopalazzo]

Recreated from #9407 (original head fork went private; identical commits, no content changes). Originally opened 2026-08-12.

♻️ Re-creation of #8984, which was auto-closed on 2026-08-11 when my fork was detached from the repository network (GitHub cannot reopen PRs closed by a detachment). Branch content is unchanged (head 84562dfed); the previous review discussion remains on #8984.

Confirmed still relevant on master (2026-08-12): plugins/sql.c has no scid() handling, scid string literals still cannot use indexes.

Summary

• When querying with WHERE in_channel='735095x480x1', the string literal was compared directly against the integer SCID column, forcing SQLite into a full table scan even with an index present
• Root cause: json_sql() passes user queries directly to sqlite3_prepare_v2() without translating scid string literals to integers
• Automatically rewrite scid string literals matching NNNxNNNxNNN format to use the scid() function before query execution, so '735095x480x1' becomes scid('735095x480x1') transparently

Fixes #8941
Changelog-Fixed: sql plugin now automatically translates short_channel_id string literals to integers for efficient index usage.

Test plan

• CI passes
• Queries like SELECT * FROM forwards WHERE in_channel='735095x480x1' now use indexes
• Queries already using scid('...') explicitly are left unchanged
• Non-scid string literals are unaffected