DigiKam's default database schema should include a composite index on `ImageTagProperties(tagid, property)` to significantly improve the performance of face clustering queries.

Background:
-----------
Current performance issue (related to separate bug report "Face tag queries slow due to inefficient OR+subquery pattern"):
- Face tag queries in the clustering UI take 10-12 seconds on a database with ~400K images
- The cause is a full-table scan of ImageTagProperties (340K rows) instead of using available indexes
- MariaDB has 6 indexes on ImageTagProperties but none on the (tagid, property) pair

Proposed Solution:
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Add a composite index to the DigiKam database schema:

    CREATE INDEX idx_itp_tagid_property
        ON ImageTagProperties(tagid, property(50));

Why this helps:
- Queries that filter by both `tagid` AND `property` can use the index for both conditions
- While the main face clustering query (with OR+subquery) cannot use this index due to its structure,
  simpler DigiKam queries that filter directly by these columns will benefit significantly
- Post-fix statistics (ANALYZE TABLE) show 25-33x speedup for other affected queries (10-12s -> 0.4s)

Performance Impact:
-------------------
- Adds ~1 MB to database size (negligible)
- Minimal write overhead (index updates on INSERT/DELETE to ImageTagProperties)
- Significant read improvement for face tag filtering queries
- No negative impact on existing queries (MariaDB optimizer will skip the index if not useful)

Implementation:
---------------
This index should be added to:
1. **Database schema upgrade script** (e.g., db_schema_XXX.sql) for new installations
2. **Migration script** for existing databases (if DigiKam version supports automatic schema upgrades)
3. **Database initialization code** in the core (if applicable)

SQL to add to schema:
    CREATE INDEX idx_itp_tagid_property ON ImageTagProperties(tagid, property(50));

Why `property(50)` instead of full column:
- ImageTagProperties.property is a TEXT column (unlimited length)
- MariaDB limits composite index size to 3072 bytes
- The longest actual DigiKam property value is ~19 characters (e.g., 'faceTagExtendedData')
- A 50-character prefix covers all actual values fully while keeping the index compact

Related Issues:
---------------
- Separate bug report: "Face tag queries slow due to inefficient OR+subquery pattern"
  (addresses the root cause of slow face queries)

Testing:
--------
This change is safe and transparent:
- Run: ANALYZE TABLE ImageTagProperties; (updates optimizer statistics)
- Existing queries continue to work unchanged
- New queries automatically benefit from the index when applicable

System Info:
============
- Tested on: MariaDB 12.3.1, Ubuntu Linux, 24 cores, 50 GB RAM
- Database size: ~900 MB
- Number of ImageTagProperties rows: ~360K
- Benefit observed: 25-33x speedup for tag-property filtered queries

Rationale:
==========
While the root cause of the main face clustering slowness (OR+subquery pattern) requires
changes to the query generator, adding this index is a simple, low-risk improvement that:
1. Benefits multiple DigiKam queries immediately
2. Requires no code changes (SQL-only schema addition)
3. Has no negative side effects
4. Should be included as a standard part of the database schema