fix(MAJORLEA-013): 6 review findings across 3 files - #74

In backend/src/main/java/cx/flamingo/analysis/service/RegionService.java around line 113, review and complete this code-review fix: RegionService.getRegionById() and getRegionByName() return null instead of Optional or throwing.
What the draft fix changed: Changed `getRegionById(String id)` and `getRegionByName(String name)` to return `Optional<Region>` instead of `Region`, replacing `.orElse(null)` with `.findFirst()` directly (which already returns `Optional<Region>`). Added `import java.util.Optional;`. The return-type change is correct and complete within this file, but callers of these methods in other files (not visible here) will now receive `Optional<Region>` and must be updated to call `.get()`, `.orElseThrow()`, or similar — those callers will fail to compile until updated. A reviewer must check all call sites before merging.
Verify the change is correct and complete; do not refactor unrelated code.

In backend/src/main/java/cx/flamingo/analysis/service/RegionService.java around line 28, review and complete this code-review fix: RegionService uses @Autowired field injection instead of @RequiredArgsConstructor.
What the draft fix changed: Removed both `@Autowired` annotations from `stateService` and `cityService` fields, changed them to `private final`, added `@RequiredArgsConstructor` to the class annotation, and removed the `import org.springframework.beans.factory.annotation.Autowired;` import. Added `import lombok.RequiredArgsConstructor;`. This is a mechanical, low-risk change consistent with Lombok constructor injection conventions.
Verify the change is correct and complete; do not refactor unrelated code.

In backend/src/main/java/cx/flamingo/analysis/service/RegionService.java around line 80, review and complete this code-review fix: RegionService.getRegionTotalPopulation() performs a full scan of all cities for every region during autocomplete sorting — O(n*m) complexity.
What the draft fix changed: Added a `private Map<String, Integer> regionPopulationCache` field and a `buildPopulationCache()` method that pre-computes regionId → total population by iterating all cities once (O(C)) and accumulating via `Map.merge`. `buildPopulationCache()` is called at the end of `@PostConstruct init()`. `getRegionTotalPopulation(Region)` now does a single O(1) map lookup via `regionPopulationCache.getOrDefault(region.getId(), 0)` instead of a full city scan. Added `import java.util.HashMap;` and `import java.util.Map;`. Risk: the cache is built once at startup and is not invalidated if cities are updated at runtime; if `cityService` data is mutable after init, the cache could become stale. Based on the visible code this appears to be static CSV-loaded data, so this is acceptable.
Verify the change is correct and complete; do not refactor unrelated code.

In backend/src/main/java/cx/flamingo/analysis/service/StateService.java around line 107, review and complete this code-review fix: StateService.getStateById() and getStateByCode() return null instead of Optional or throwing.
What the draft fix changed: Changed `getStateById()` and `getStateByCode()` to return `Optional<State>` instead of `State`, replacing `.orElse(null)` with `.findFirst()` directly (which already returns `Optional<State>`). Added `import java.util.Optional;`. The return-type change is correct and complete within this file, but callers in other files (not visible here) that previously received a `State` directly will now receive `Optional<State>` and will fail to compile until updated. The reviewer must locate all callers (e.g. via `getStateById(...)` and `getStateByCode(...)` usages across the codebase) and update them to use `.orElseThrow(...)` or `.orElse(...)` as appropriate. This is a breaking API change that cannot be fully resolved in this file alone.
Verify the change is correct and complete; do not refactor unrelated code.

In backend/src/main/java/cx/flamingo/analysis/service/StateService.java around line 72, review and complete this code-review fix: StateService.getStateTotalPopulation() performs a full scan of all cities for every state during autocomplete sorting — O(n*m) complexity.
What the draft fix changed: Added `buildPopulationCache()` called from `@PostConstruct init()` after `loadStates()`. Added field `private Map<String, Integer> statePopulationCache` and method `buildPopulationCache()` which iterates all cities once and accumulates population per `stateId` using `Map.merge`. Changed `getStateTotalPopulation(State)` to do an O(1) `statePopulationCache.getOrDefault(state.getId(), 0)` lookup instead of a full city scan. Added `import java.util.HashMap;` and `import java.util.Map;`. The cache is built once at startup; if city data changes at runtime the cache would be stale, but given the existing `@PostConstruct`-only loading pattern this is consistent with the rest of the service.
Verify the change is correct and complete; do not refactor unrelated code.

In backend/src/main/java/cx/flamingo/analysis/service/LanguageService.java around line 62, review and complete this code-review fix: LanguageService.getLanguageById() returns null instead of Optional or throwing.
What the draft fix changed: Changed `getLanguageById(String id)` return type from `Language` to `Optional<Language>` and replaced `.orElse(null)` with `.findFirst()` directly (line 62-65). Added `import java.util.Optional;` at line 10. The method itself is now null-safe. RISK: Any callers of `getLanguageById()` in other files (controllers, services, etc.) that previously used the returned `Language` directly will now receive an `Optional<Language>` and will fail to compile until updated to call `.get()`, `.orElseThrow()`, or similar. Those callers are not visible in this file and must be updated separately. A reviewer should search the codebase for all usages of `getLanguageById` before merging.
Verify the change is correct and complete; do not refactor unrelated code.