add tag indexes

[pjfanning]

Issue 1: Missing Tag-Leading Index on event_tag Table

Problem: The event_tag PK is (persistence_id, sequence_number, tag) — tag is the 3rd column. eventsByTag queries filter on tag first, so the PK index can't be used efficiently.

Fix: Added event_tag_idx index on the tag column across all 6 database dialects:

│ postgres/postgres-create-schema.sql │ CREATE INDEX CONCURRENTLY event_tag_idx on public.event_tag(tag) │
│ mysql/mysql-create-schema.sql │ CREATE INDEX event_tag_idx on event_tag (tag) │
│ mariadb/mariadb-create-schema.sql │ CREATE INDEX event_tag_idx on event_tag (tag) │
│ oracle/oracle-create-schema.sql │ CREATE INDEX EVENT_TAG_IDX ON EVENT_TAG (TAG) │
│ sqlserver/sqlserver-create-schema.sql │ CREATE INDEX event_tag_idx on event_tag (tag) │
│ h2/h2-create-schema.sql │ CREATE INDEX IF NOT EXISTS "event_tag_idx" on PUBLIC."event_tag" ("tag") │

Scala/Slick: Added val tagIdx = index(...) in JournalTables.scala:123

Migration scripts (for existing databases):

• postgres/postgresql-event-tag-index-migration.sql
• mysql/mysql-event-tag-index-migration.sql
• mariadb/migration-2.0.0/mariadb-event-tag-index-migration.sql
• oracle/migration-2.0.0/oracle-event-tag-index-migration.sql
• sqlserver/sqlserver-event-tag-index-migration.sql

Issue 2: Missing global_offset Index in Oracle durable_state DDL

Problem: PostgreSQL, MySQL, MariaDB, H2, and SQL Server all had state_global_offset_idx — Oracle was the only dialect missing it.

Fix:

• oracle/oracle-create-schema.sql: Added CREATE INDEX STATE_GLOBAL_OFFSET_IDX ON DURABLE_STATE (GLOBAL_OFFSET)
• Also included in oracle/migration-2.0.0/oracle-event-tag-index-migration.sql